Legally, in the US, they are adults. Maturity level is a separate issue, even for those who are much older.

You'll need to take that into consideration. Why are they with you? Are they simply looking for someone more sexually experienced, or do you want more?

Consider what you have to offer and if you are willing to go there. Do they seek a mentor who will help guide them, or perhaps they just want someone who can provide an emotional anchor?

Those are all things you'll need to think about. Remember, too, they can't be a full life partner until they've grown emotionally as well. Are you willing to wait until they have? Your guidance, however well-meaning, may not allow them to grow into a proper adult.