Hi. I've been running in Vibrams for a little over a year.
I don't think there are any age (I'm 57) or weight limits that are any different from running in general.
But--and that's a really big BUT--you need to start with very low distances and pay incredible attention to form. Land very light. Concentrate on short, rapid, springy strides, and don't land on your heels. Just having the feedback from the bottoms of your feet helps with form, but you should pay conscious attention as well. (When I first started I found I had to chant "light, light, light" under my breath to remind myself.
The best advice I ever had was from someone who told me to run as if you were trying to sneak up on something--soundless, quick, short paces.
A lot of people overdo it and end up with calf overstrain, Achilles problems, top-of-foot pain. Your legs and feet are used to shoes, and so all that connective tissue and muscle is used to being shortened because of elevated heels. You can't change that overnight...or even in a couple of years.