Not entirely sure what your question is?
Acer is an enormous genus with a lot of variety.

Maples can self pollinate, be wind pollinated, or pollinated by insects. Unless they are grown in strict environment there is no way to prevent this from happening.
So if you have a "A" in your yard and I have a "B" in my yard, the seeds on both our trees could be crosses or could be not.
No way to know.
Really unable to know what other maples are in the neighborhood.

What we do know is that seeds from an "A" do not grow unto baby "A's." The seeds do not necessarily grow true to the parent tree because there is no way to control pollination- they literally could be anything. A seed is by and large just a generic JM and will never grow into a named variety.

Most Japanese Maple varieties you see are grafted- grown from cuttings.
If you want a genuine "A" you either have to buy one or graft it yourself.

Trees generally begin flowering at "maturity" which can be anywhere from 8-10-12 years old depending on the conditions in which they were grown.

Here is a very good, educational page discussing myths about JM's.

Here is a link that might be useful: Good growing guide

Thank you soo much for helping me better understand. You answered my question.
Just to make sure I am understanding though, some never produce seeds? Or is it that they just aren't being pollinated?
I see what you are saying though about controlling the environment. There are several red JM in this community and many different types. And am I correct to assume that they can be pollinated by any maple not necessarily a japanese?
We live surrounded by forests and many "helicopter" bearing trees. I'm really a newbie here so bear with me.If those are maples, the JM can pollinate those and those can pollinate the JM?
And the different types of red JM can cross pollinate as well?

I don't know what species and/or varieties of Acer's cross pollinate.
Hopefully someone with more knowledge will stop by.

And yes, some may never produce seeds and other produce sterile seeds.

Also be aware that seeds from red trees will often produce green seedlings- this is why we say seeds don't breed true.

Ahhh, the sex life of Japanese maples!! It is complicated to be sure :-) They can produce just male flowers, male and female flowers or even bisexual flowers, but not just female flowers - the botanical term for that is andromonoecious. So technically a tree can be self-fertile - that is, pollinate itself. Or it can be pollinated by another Japanese maple with the appropriately sexed flowers. They will not be pollinated by any other species of maple.

Here is a link that might be useful: tree sex

Thanks!!! You guys have really been helpful.
I believe the dwarf my brother & fiance have is the Acer palmatum dissectum. And I believe the one I just purchased is the Acer palmatum 'Atropurpureum'. Of course this is just me guessing.
So far neither my brothers nor my fiances are seed bearing, what a bummer - I had hopes for a free seed!
The one I just purchased came from a seed bearing parent but I'm still at a loss for its mature age.
So as I read your link gardengal it seems we all probably have monoecious trees and none will ever bear viable seeds since none are very near another JR maple.
What I am understanding is that with these particular varieties I would need two trees; a young monoecious one that would be female and a more mature monoecious one that would be male.
But without the mature age of these trees I wouldn't know where to begin. So the hopes of owning a fertile seed bearer are pretty much shot :( .
However, the seeds I have been given sound like the Acer palmatum 'Atropurpureum' and were picked two days ago. I'm not sure if I should attempt to grow them since I don't think they were/are ripe. But I guess there's no harm in seeing what happens.

Of course the more I read the more they are all similar to the acer palmatum dissectum tamukeyama. What do I know. I cannot seem to find the age at which these lil guys are mature though. So it seems none of us will ever have seeds, which is sad because when they produce the seed the colors are that much more beautiful!

Sexual maturity will vary with tree - since all named forms of Japanese maples are grafted, it is not at all unreasonable that a very young and quite small tree can produce viable seed, as the scion could be from a very mature stock tree. Seeds must be ripe before they will germinate, so collecting now is just a waste of seed - they need to ripen on the tree. I'd do some research on growing JM's from seed - there are stratification requirements that should be met to achieve good success but searching through some of this forum's archives will produce some very good past discussions on this topic. Personally, I think the easiest way of growing JM's from seed is just to collect sprouted seedlings from under my mature trees and pot them up :-)

Without seeing the flowers - and therefore sexing them - on your individual trees, there is no way of knowing if your trees will produce viable seeds or not. FWIW, the species and generic catchall 'Atropurpureum' are very free seeders, which is why seedlings of these are so commonly used as rootstock.....cheap and readily available.

Thank you sooo much for all your insight! You really enlightened me to things I hadn't even considered. Here I am getting frustrated not understanding why I cannot find anything that lists an age of maturity, never thinking about the grafting aspect affecting their age.
This all started with getting mom one for mothers day and then we all thought in a few years we could exchange seeds and share our varieties, boy was I wrong! lol.
Part of me would love to run out and buy a mature tree to showcase all its beauty. But watching it grow is most of the fun, for me anyhow. I suppose we could all consider attempting to swap cuttings and use some root growth hormone to exchange varieties. Because I love the small bushy ones that cascade.
And hey if any of my trees turn out to bear seeds I'll be happy to return here to share away!

Sweetybyrd, This article was written by Brad who many years ago used to frequent this forum, but know stays mostly on UBC. He is truley a pro and if you need more to chew on read this. Hope it helps....AL

http://www.fujiyamagarden.com/page19.html

I would throw away the seeds your friend brought you from a neighbor of his. They aren't mature enough yet to germinate.
If they are old seeds from last year, the're probably infertile. Good seeds drop off in the Fall. Seeds should be collected just before they drop.
Mike

I should also add that in my experience, seeds from the small red, weeping forms, have a very low rate of germination. Named Japanese Maple cuttings, weeping and upright, are almost never done. They are usually propagated by grafting.
My maples are propagated by seed. They are almost worthless on the market, but I'm not selling them so I don't care. I compost all but the best.
Mike

Here is a link that might be useful: Fall slideshow of my garden

WOW !!! Truley amazing Mike! Thank you for sharing, breath taking, who wouldnt love to drive down that driveway everyday. the laceleaf is unbelievable, perfect! and the weeping are beauties. I cannot get over the rhododendron and was that a blue spruce? Blue spruce is on my list, they are something else. And that elm, wow you really have a spectacular garden. I'd love to have the room for the full sized trees but right now I have a little yard and stick with small transportable plants that I will take with me one day into my dream yard/garden. You really have done an incredible job with the colors and sizes. I've never heard of the blue star but it definitely caught my eye. Stunning! Congrats!