The older you get, the less the age gap matters.
This has been discussed here before. The main concern with age gaps is this:
What stage of your life are YOU at, and at what stage of life is YOUR PARTNER? The concerns that OFTEN (but not always!) disrupt relationships with age gaps stem from this issue. People aged 18-25ish are generally in a mode of self-discovery. As a legal adult, you are now free to ignore the influences of your parents if you wish, and set foot into the world! You get to learn about people on your own terms, and make decisions about who you want to be on your own terms. As a result, collegiate-aged people are very fluid and dynamic. You can be a first-rate student at 19, a disaffected liberal at 20, a paranoid recluse at 21, a grease monkey at 22, and by 23, you're finally a vet-tech or math teacher or rock bassist in a garage band or full-time parent or starting your own business. It's a very chaotic time. It's not until our mid-20s (generally speaking) that we start to 'settle down' and streamline our identity (which is generally often very much defined by our occupation).
This is generally why the age gap between someone who is 28 and 44, while uncommon and unorthodox, is commonly viable long term. Both partners have settled into their adult identities. The gap between a pair who are 19 and 30, on the other hand, is smaller, but creates a potentially more volatile situation, as one person is well settled (generally speaking) and the other is still establishing who they will be (generally speaking).
This is not a hard and fast rule with no exceptions, obviously, but it is, generally speaking, how things tend to be. *shrug*