I have to echo the points re mortgages. I was in my late 20s (and got my first card around 19, so less than 10 yrs of history) and had a score in the high 700s, with no mortgage. By the way, I got my mortgage in early 2011, and got into the cc game in mid 2011. I went to refinance in late 2011, after applying for a bunch of cards, and not only did I easily get approved for my refi, my credit score was basically unchanged (+/- 10 points).

So, (1) you don't need a mortgage to get cc's, and (2) having lots of cc's won't affect your mortgage eligibility.