Is there a age limit to become a CFI?
A flight school won't care too much about your age if you have the required medical.  And mentally, your age is an overall plus since a flight school would generally prefer a mature individual in it because they want to teach to a 25 year old who's just building time.  The turnover with flight schools is quite high so the prospect of getting say at least 5 years out of you is another plus because they may be only getting 2-3 years out of younger instructors in the current environment (often they are basically only hanging on to get to 1500 hours under the "Colgan Rule" craziness).

I would say that that if your health is fine and shows no signs of problems over the next 5 years at least, you should definitely go for it.  You might even get a school to commit to hiring you, or at least giving you a shot (it's going to come down to how good a teacher you are), if you train with them.

With the general shortage of bodies all through the system, I don't see any downside if you really keen on doing it.  If you gain a reputation as a good teacher students will gravitate to you over the youngsters.