Not caring about your age, or the age gap, comes with maturity. I'm not neccessarily saying you're immature, but the older you get, the less age will matter.
Just make genuine comments about how close you feel to her, how much fun she is, how you love how well you two get along and so on.
I suggest just being confindent and secure about who you are. That's what will make you attractive to her. She might have to think it over and face her own fears about what other's think, but keep pursuing the friendship and eventually, you'll win her heart.