I've been married twice: once for 7 years to a woman who was 7 years my senior (I was not a Christian), and once for 15 years to a woman who was 11 years my junior (I was a Christian).
Age DOES make a difference. Oh, not in whether you can find things in common, or whether you can love each other - but you go through life changes in your mid-20s and again in your mid-30s. I am in my mid-40s now and someone else is forcing me to go through a life change I'd like to avoid (divorce).
But I ended up basically raising my second wife, and I think my first wife basically raised me. I WAS immature. There's a lot of patience needed for the older person in the relationship. Plus, its possible that you will have many morals that are different and many different expectations, simply because of your age difference.
Are these differences insurmountable? I dont think so. But, then again, maybe so for some people. There are a lot of issues involved, but my second wife is divorcing me after 15 years and 4 kids in part because she is going through a "mid-life" crisis. If you are going through life changes at different times, you have to be patient with the other one and work extra hard to make a marriage work.
As for 15-16 year olds dating 18-20 year olds, its creepy because BOTH of them are still children, and in the USA at least, one is legally a child still.
One should not date until one is ready to get married, and then it should be used to find your spouse. Dating for any other reason sets you up for sin (temptation at the very least).