the reason might be that the young generation have more knowledge about digital environment , digital currencies and understand the functioning of blockchain based crypto , they are more openminded and willing to change ,
plus investing in bitcoins are less hectic and comparatively easier to invest in as compared to stocks or other investments which require regulatory procedures .
The young generation might be aware of digital currencies but i doubt that majority will understand the technical aspect of blockchain and you really do not need to know the technical aspect to invest in them, just the basic of trading is more than enough to be an investors.
now in most of the countries people below age 18 cannot have a bank account but if they want to start trading they can just borrow some from parents and invest in any crypto without and hassle .
Users below 18 should not invest their money trading and once the exchanges enforced mandatory KYC i am sure they will not allow anyone below 18 i believe to trade in their platform.