If you have a passion for fitness and want to help others, then I say go for it. You may find that there is a great market specifically for your age group. I am 47, and find that many people my age want a trainer who understands and can relate to our age group–both with regard to physical changes and social aspects. The baby boomer age group is also wanting to take care of themselves, and there is a bigger market for them right now. However, you could find your niche in any age range of individuals.
Again, if you have the passion to do this, I say go for it! There are trainers out there for every kind of client.
Best wishes to you,