Personally I think that is frightening at that age. I think its scary how quickly and easily people can be loaned money through loans, credit cards and higher purchase these days.

The day I was 18 I had all the papers ready to sign for our home loan. We built a house and for the first part we went really well. But we had no idea how to manage money and refinanced so many times in such a short period it wasn't funny. We had parents guiding us as well but hey, as if we were going to listen to them!

It taught me a massive lesson and I'm sure your SDD will learn as she handles it all. Def provide her with all the tools she will need to budget etc. As for her buying a big car that frustrates me lol as its not a smart decision. But again its her choice. Good luck to her!