I started paddling at 16 and am still paddling in my late 50's. It was an obsession with me, that has been central to my life, in that my circle of long term friends grew up around kayaking, and it is still very much at the core of who I am and what I do. I had a few years out in my early 30's due to a shoulder injury, but getting back on the water was my motivation to getting it sorted, which eventually happened courtesy of the NHS. It was hard to get back up to the previous level of fitness and confidence as well as rehabing at the same time, but worth it. If my paddling was getting stale I got into another branch of it, to give me a new challenge, such as OC1 and trying to do the same rivers I had kayaked before. I still paddle G4 and am happy to walk round things, and I accept that I am not willing to risk injury, as I take a lot longer to heal now. I used to feel less positive about not doing some rapids, but now tend to just enjoy the fact that I am out on a regular basis sharing the pleasure of what I do, in the company of similarly minded paddlers. I do enjoy the buzz of paddling G4, and sometimes a little more, but its all normally within my comfort zone, with just the occasional tip toe out of the zone.
Managing the mental game has been interesting as it does form a more evident barrier as I get older, and changing the format is working for me. I recently got back into kayak surfing and now get a buzz from dealing with big waves (for me) as I am more comfortable at pushing this as I feel that I can more readily deal with the consequences of a big wave messing with me, then getting worked in a stopper.
When I read the title of this thread my first reaction was to think whats the issue as I would be delighted to be in my thirties and have to deal with this problem, but I dont see kayaking as a young persons sport, its far more inclusive than that. I sometimes paddle with a stalwart of Scottish paddling who has recently turned 70, and to celebrate this we had a week in October paddling the Spean, Roy, high Orchy, Etive and Coe, the group was made up of quite a wide range of ages, so in my view it it is very much down to personal motivation and enjoying the activity, and company of others, to get you out and into the world of paddling.