Most riders will tell you it all depends on your horse, their upkeep through life, and their soundness. If the horse can do it physically and mentally wants to, keep going.
I am in the same position. I have a 14 year old Hano X who is starting Training Level, I don't know if he has the scope for much more. We may hang out here till he's settled enough mentally to pack someone else around the lower level.
I would lean toward an OTTB as an event horse. Not saying the Paint can't do it, but OTTBs just have a wonderful natural gallop. Perhaps start lessoning/schooling on some OTTBs so you can get a feel for their run and jump style. You have some time. Start building connections and getting the experience you will need to be successful now and with a future horse.
If your horse says no, you either asked the wrong question or asked the question wrong
And God took a handful of southerly wind, blew His breath over it and created the horse