We see Harrison drinking and gambling in the series, so one of the following is probably the case:
1) We're told that Tru is 22, though this was at the start of the series. It's possible that (like Eliza) she is now 23. If Harrison is 2 years younger, that would make him 21 and make the drinking and gambling thing a non-issue.
2) The series (much like Real World, etc.) doesn't care too much for the law and understands that underage drinking and so forth happens. He's shown at bars where he should be carded, though maybe nobody thought about it or cared.
3) The city Tru and Harrison live in is s'posed to be in Canada, like where it was filmed. I think in the episodes they mention states and imply "large American city", but in Canada, Harrison could legally drink at 19, meaning he could be 20. I have no idea about gambling in Canada and I dunno if 19 is the drinking age there universally or not (I know it is in Windsor and I'm fairly sure in Vancouver as well).