Riding a bike in Cent. America, age does not matter. Do you have any sort of driving license at all? For Latin America, any license will do, car or bike, does not matter. Main thing you need if pulled over would be your TVIP and Passport.
No police will know what your license is. Passport is key.
125cc bike is fine if you don't mind traveling at a slower pace. Much of the PanAmerican is crowded with truck traffic. Passing safely on a 125cc bike could be risky. Sitting behind 5 Diesel trucks belching smoke is NO FUN!
On rural dirt roads the 125cc bike would be good. So once away from main roads the 125 will be easier. In traffic in cities it will be good too.
Most countries there allow foreigners to buy a bike locally, but some may have restrictions on crossing borders ... so check into that to be sure you can travel through ALL countries. To sell the bike off at trips end my guess is you will have to return to country where you bought the bike.