The only thing you can do is using social engineering besides asking nicely. There is absolutely no way for imvu to verify a user's age unless they actually buy the option from the shop and confirms it with papers, id cards, passport, driving license.
Now in 75% of the cases you can make an approximate guess with 3/4 years fault, look up the account date creation, and take in account that people start using imvu at about 14-18y/o add the account seniority and you can round up something closer to their real age.
someone with over 10 years of imvu, I promise you will not have under 20 years.