Most are probably on their 30s and 40s, especially those with prior interests in cryptography and a digital cash system (Nick Szabo, Hal Finney and others, just to name a few). While the majority of those with interests in crypto are in their 20s, it's undeniable that those whales are somewhat older, due to their money and experience in the field. It'll be cool to know the demographics of bitcoin users but then again, that goes against 'privacy' and some statistics regarding age groups of bitcoin users may somewhat be unreliable.
This is probably the correct estimation, in order to recognize how great bitcoin was just after it was created you needed to be looking for something like bitcoin for quite some time, you needed knowledge not only in cryptography but also in economics and the history of the world and getting that amount of knowledge will not be easy for a kid or a teenager so most likely the average age of long term holders is above 30 years old.