Same age can still be abuse.
I was 4 or 5 when I first experienced abuse and the individual was actually a few months or so younger than me. However he definitely knew more, had experienced more etc.
I also struggled for a LONG time to acknowledge it as abuse.
What then REALLY got confusing for me, was as we got older his younger siblings (1 sister and 1 brother) also got involved and by the time I was 11 or 12, and his younger brother was 9, he (you younger brother) was teaching and doing things things to me that I couldn't have dreamed up. I can easily see now that the kids in that family were all abused by someone and they were simply acting out with me, the damage though is still there, and it still qualifies as abuse, even though their intent wasn't to hurt someone.
That being said, because of that, I have been able to easily forgive them, 1 because of our really young age and and the fact that at that age we had not clue about it.
Unlike the last two who were older and knew better and the last one downright forced himself on me.