What are they?
They are aircraft, boats, cars, etc. that are controlled in their movements by radio signals from a transmitter, usually hand-held. They vary from simple to highly complex in nature.
What do you do with them?
You operate them, hopefully in a proper manner. There are groups for the pleasure of operating them and for competition.
As to ages, and types of vehicles.
That is a matter of the interest of the person involved.
Especially with aircraft, they should start with "trainers" as they are much easier to fly than more advanced models. ALL flying models of "war" aircraft, other than toys, are advanced models!
Being more familiar with aircraft, I can tell you that IF they develop a good interest in them, they are likely to progress from "Ready to fly"(RTF) models, to building from kits, to building from plans, to possibly designing their own.
The building of more advanced models requires and/or develops manual dexterity and flying develops "fine muscle" co-ordination, as well as practical knowledge of how and why control systems work, although they will NOT learn much about the radio electronics due to federal rules regarding this equipment and its use and/or modification, (to help prevent causing interference!)
Be aware that MUCH of what is sold in "toy" departments are just that, TOYS, and NOT meant for those with a serious interest in the field.
Hope this helped.