I heard that in Costa Rica, if you are 16 and have a foreign licence, that you can legally drive a 100cc motorcycle. Is this true?
This official website states:
Todo Conductor (Costarricense o extranjero) podrá conducir los mismos vehículos que autoriza su licencia extranjero por un plazo de hasta tres meses contando a partir de su ultima entrada a territorio nacional. Durante este plazo NO se homologaran licencias.
Which roughly translates as:
Every driver (from Costa Rica or foreigner) can drive the vehicles authorized by his foreign license up to a period of three months counting since his last entry to national territory. During this time licenses will not be certified.
So in principle it seems that you could if you're staying for less than three months. If you're planning to stay longer you would need to certify you license.