My father was 15 years older than my mother. It didn't seem like that big of a span when I was young, but as they got older, I noticed she was still young and he was starting to seem like an old man. Plus he got sick in his mid 50's, was bedridden, and she became more of a caretaker than a wife at a young age (which I know can happen at any age, but it is more likely when there is a large age span between the two).
I have a SIL who is about 12 yrs younger than my brother. He is retired but she still has several years to work before hitting retirement age. They should be traveling and doing things together but their age difference is preventing it.
A daughter of friends just married a man 15 years older. She is in her early 30's; he his late 40's. They just had a baby. He is going to be putting a child through college when he should be enjoying retirement.
As you age, so does he. The difference becomes much more noticeable. I personally wouldn't recommend such a large gap.