Today you are however old you are.
You may be in your teens or your twenties or your thirties or more but this year is your year to do with what you want.
It's taken years to become all that you are. It's taken the places you have been and the people you've encountered and things you have learned. Here you are though; the product of choices and luck and effort.
It doesn't matter how old you are; it matters that your age is your age. You have done things and seen things and you know what you know. You know that there will be new things in your future (and many of the same things too) and you know that they will change who you are.
Right now though, you are your age. You can make this year a good year by making each day a good day. You can look back at where you were and how far you have come. You can imagine how your decisions today will lead to amazing things.
Much like a river, you are always the same and you are ever changing but today you are who you are, where you are, and when you are. Today you are you.
Happy belated birthday and happy early birthday! I hope all the days in between and good ones.