Building Credit - How am I doing?

I just paid off my main credit card, and requested and received a credit limit increase, from $1,500 to $5,000. I also applied for and was approved for a second credit card, with a limit of $5,000. My only other debt is under $1,000 on a jewelery store credit card.

I have very little credit history, but my score is good, according to creditkarma.com (how accurate is that, does anyone know?), around 750. According to the website, I can make up to 4 more hard inquiries into my credit before it will negatively affect me.

One of the negative things affecting me is the short average age of my accounts. I know opening that second card will negatively affect that further. However, I figured the available credit/used credit ratio would balance that out a bit.

My main question at this point is whether I should apply for another credit card, to further expand my available credit, that way I can sit back and wait for the accounts to age, or if I should let this new one age, and then open a new one some years down the line? I'm worried that if I go with the second option, I'll be stuck in a sort of yo-yo pattern, where I keep on raising and lowering the average age of my account. Seems to me I should open my intended amount of credit cards, and then stop and let them age (I plan to have one bill automatically paid by each credit card, and I'll pay off the balance each month).

What do you think?