Last year was the first year for my then 5 yr. old Grandson on a towable. I have an inflatable Jet ski that everyone likes so he used that. It's a lot of fun for young and old.
I think the biggest issue is that the child has to feel comfortable in deep water and know they won't sink and drown. I insist that before my Grandkids can go on any towable that they take swimming lessons so that they are
comfortable being in water over their head. I believe that once you give the kid the confidence that he will be safe, you will see little fear from them. I would never put anyone on any towable until they proved to me and themselves that they were ready and able to deal with the deep water.
Naturally a proper life jacket is required. When I say proper I mean proper size and fit as well as proper style and condition. Normally for this type of activity I believe a USCG Type III is recommended but you would have to check with the Marine Patrol to confirm that.
Have a great time on the lake. Be Safe and happy while you enjoy this wonderful resource that we call the lake.
CT