Is age really just a number?

When I see a fifteen year old with a nineteen year old I think to myself what could they have in common? Normally a teen at the age of fifteen should be in the tenth grade and someone who is nineteen should be out of high school and in college or have a job. Also someone who is nineteen should be way more mature than your average fifteen year old. This of course means that the nineteen year old will have a huge influence on that fifteen year's old's decisions and if they so choose take advantage of that fifteen year old. Basically a fifteen and a nineteen year old are not on the same playing field which means it wouldn't make sense for them to date.