Certainly not. I don't really think any age really is to old in earning their HS diploma. Sometimes teens who's parents may travel a lot; credits aren't transferred. It's unfortunate, but the important thing is that you graduate! Nice work so far. Your all good! However if you turn 27 & write this again- chances are my answer will be different. Good Luck!
You're a little old but it's not a big deal. My brother is old for his grade but his birthday is in December, and it's my understanding that the sort of cut-off date for age in school is in December. So I don't really understand how you ended up being in the grade that you are, when I have a friend whose birthday is in November but she's YOUNGER for her grade (so if you two were in the same grade, you would turn 18 before she turned 17 is what I'm saying). Eh there's really no use in worrying about it now.