This is where the problem comes in for me, and maybe it's just my issue and not yours. For me, it is hard to separate a current adult/adult relationship from the residuals of an adult/child relationship that previously existed. When a young person often seeks the advice of an adult, that puts the adult in a position of power that I don't necessarily think time and maturity completely take away.
In thinking about a couple of people now well into adulthood that we advised and mentored as young teens moving towards adulthood; if we were approached by any of them with the same proposition, we may be tempted, but we'd most likely decline. For us, it just wouldn't feel right. I'm not saying that's the right answer for you or anyone else, but it would be the right answer for us.
Good luck with your choice and outcome.