As far as shared accounts, both kids have been authorized users on one of our credit cards since age 17 or 18. They used it for groceries, medical and dental appointments, textbooks. They asked our permission before buying a t-shirt or word processing software.
We had one shared bank account with only one of our kids. Husband qualified and was able to assign his post-9/11 GI Bill benefit to DS2. (DS1 had graduated by the time DH was eligible.) Money was going for tuition etc., so son first said to have it deposited in one of our accounts since we took care of the payments. The government, rightly, requires that the payee be named on the receiving account, so son said joint account. We ended up with DS2 as recipient, DH since the benefit flowed from him and he could pursue any problems should they crop up, and me since I was the one with the login at school’s accounting office. It was a bigger risk for DS2 than for us; the money was/is his, after all, and there we were with unlimited access to it.