Might be water sitting on the leave...?
Or Sun burn. ..?

Where is it?, Do u got a photo?
Even a pic of after u cut it off

There is the possibility that it's just dying from "old age," in the sense that no leaf lives forever.

I'm pretty sure your Agave will be find outside in the summer in zone 6.

What about your care of the plant? How often do you water? Is it potted in plastic or clay? What type of potting mix is it in? Just a few questions to help determine if the yellow leaf was caused by another factor other than age.

Planto

I do water once a week and it is in a clay pot,the mix is cactus medium and it currently sits on a table in front of a window that gets afternoon full sun. I will post a pic later this afternoon.

thanks

When you say cactus medium, what's in it? My "cactus mix" is different than the next person and so on.

I understand that it's getting afternoon sun, but do you know if it's a South window or a West window?

South is best for cacti and succulents being grown indoors, on the windowsill is absolute best (the further away from the window you get, the less light the plant receives). I realize sticking an Agave in the window wouldn't exactly work... that's why there is the option to stick it outdoors in the Summer (you could probably do it in the Spring after a certain temperature has been maintained). I don't know the minimum temperature for Agaves, I believe it depends on the species or even the cultivar.... do you know which Agave you have?

Just trying to think of all the different ways that we can help your Agave meet it's "full potential." In other words, find the ways to make it most happy.

Planto

The Agave is planted in Miracle Grow Cactus Soil and sits in a window facing West,here are some pics and yellowing arm

Okay, I can give you two recommendations. Clay is good (plastic is okay too, but it doesn't dry as fast).

1. If you're going to keep it indoors, can you place it in a South window? That would be better than a West window.

2. MG Cactus soil isn't the best potting mix out there, but it's what I use too. However, by itself (not amended), it's crap honestly. There's very little perlite in it, and it's not fast draining (like the bag says). There's nothing wrong with using it IMO, but I've amended mine with rinsed perlite to make it faster draining. About 50/50 is a good mix.

How long have you had it? How big is the pot to roots ratio? Just want to make sure it's not overpotted (that along with the unamended MG soil can create issues).

I believe you can put it outside if it's warm enough and the possibility of frost is gone. What are your average temperatures right now?

I hope this helps.

Planto

I have had the plant 4-5 years and it is in a pot measuring 7" inside diameter

Does the root system fill the pot?

Planto

As Beth said, it looks overpotted to me, too, as well as having soil not porous enough. Her words are righteous and will get your Agave on the proper path for living. It will do fabulously outside over the next few months - you just want to acclimate it over a week or so to full sun.

Hard to tell if the root system fills the pot don't really know how to tell ,however is the consensuses that it should be in a smaller pot?? Maybe 5" pot it is in a 7" pot now