would you automatically assume she had lots of money?

not at all because infinitis are not that much. I guess it only depends to certain people because little things can impress certain people. People that have more money than the average person doesn't impress them at all. Now if it was a S550 or a BMW M6 than YES thats impressive for a 21 year old.

Like me, I have an AUDI S4 convertible paid off and im only 23 but doesnt mean I have lots of lots of money, but my BF and family does LOL

Yes. I would consider her fairly wealthy. That's going to run her over $30,000. At the age of 21 most families with a middle class income would be struggling to pay college tuitions, not buying their precious daughter a new luxury vehicle. Cars most certainly are a direct display of wealth and social status, anyone who disagrees with that is foolish.

Honestly, I really wouldn't care. I don't think the kind of car you drive defines you as a person.