Starting at around 20-21, an age difference of up to 10 years isn't that big of a deal.
Pre-20 however, is a big deal. Just the way things work.
The biggest age-difference-matters time of your life is highschool, where even 2 years might get a little iffy with people. ESPECIALLY if a person in highschool dates someone who's already graduated. Like an 18 and 20 year old. Gets a bit sketchy there, but that's only peer wise of course. It doesn't REALLY matter in the grand scheme of things, it always comes down to how much value you place in others opinions of you.
Time knowing each other matters quite a bit. A lot of people (ages 21-) mistake infatuation for love. Big mistake. Gotta make sure you're past the infatuation stage before you can make a valid assessment of whether or not you actually love the person.