One thing I've wondered about if someone were to actually succeed at making themselves look and feel 18 is how our legal system would react.

Imagine trying to buy a house when your face says 18 and your ID says 78.

Getting a drink at a bar? Showing ID to vote? Withdraw money from that 78 year old's account? Collect social security?

You're going to get denied at best, and likely accused of identity theft/fraud.