Lots of good info on this thread already, I'll just add an anecdote:

I recently went through this exercise with my 19 yr old Daughter. She has had credit cards before, one as AU and one as primary. She also has had a debit card for probably 3-4 years. We cancelled the credit cards when they were up for renewal, and she started using one of ours, which was a pain doing constant reimbursing (in particular seemingly constant use of Uber).

So I helped her apply for Citi DoubleCash over winter break but to my surprise she was denied. I assumed since she'd had credit cards before she would be considered credit worthy. So I created an account on one of the credit report sites, and it showed that her score was something like 640- too low for the DoubleCash.

So my next move was to add her as an AU to one of my cards. This worked initially, and her score jumped up to a ~720. Unfortunately I chose a card that I often maxed out and had a low limit- 10K. The next month her credit utilization went to ~90% and her score plummeted to ~620.

So I removed her as an AU from that card, and added her to another card that I rarely use. It took about 2-3 months for the old card to drop off and the new card to show up. Once this happened, her score went back up to 700+. We reapplied for the DoubleCash and she was approved.

So if you are using the AU trick, make sure that you use a card that has little use and as high a credit limit as possible. Also from what I can tell the account age does factor into the average age of credit component of the credit score, so add her to one of your older cards if possible.

Her debit card does not show on her credit report. Interestingly, each yearly Stafford loan she's taken out for college does show up- it will impact average age of credit negatively for a while, but there's nothing you can do about it.

I thought about having her apply for Chase cards due to 5/24 but she won't put much spend on her card and I feel like she should probably wait until she it out of school to really get into miles & points if she is interested. She'll be under 5/24 by then I hope.

Good luck!