how old is it?

I can only guess that perhaps some damages may have occurred in the trunk above from where the offshoot. I'm not aware of what kind of disease sumacs are susceptible to but observe the trunk and look for insect infestations or fungal or bacterial problems. The offshoot will probably emerge as a new trunk so keep it healthy. Don't overwater.

Hi There, Thank you for your responses. I don't know how old the plant really is. However, I moved in the house 2 years ago, and I know the previous owners did a major landscaping overhaul in 2004, so if I were to guess, I would guess 6 years old. The odd part is that there is no signs of infection or insect infestation what so ever. The other Sumac that I have is standing tall only 10 feet away. The off shoot that I mentioned earlier, is growing straight from the base of the main trunk, so I doubt that the root is dead...Do you think there is a chance that the leaves will grow back this summer, or next spring?

If it's still shedding all it's leaves, I think perhaps the old trunk is dying but a new one is growing. So maybe add a tree fertilizer to keep the young plant growing healthy. Sumacs do produce offshoots when their older one dies off so perhaps this is one of those moments. Scratch the trunk of the old one and look for a vibrant green colour. If it's there, it's alive. If it's not there, it's dead. If it's spongy, it's dead.