Age in Harry Potter-(precis)

One thing that intrigues me about the Harry Potter series is the extreme age difference of the characters. When Harry and his friends are accepted into Hogwarts they are 11 years old and their journey continues year by year until in the final novel they are 17. This didn’t bother me, because it was like they were growing up with me. By the time I started reading the books, I was about the same age as Harry and his friends.
The thing that did throw me off balance was when I realized the extreme age of the adult wizards in the series. The fact didn’t occur to me until I had reread the series a few times, but I now realize that the majority of the adult wizards are actually quite old. For example, in Harry Potter and the Chamber of Secrets, we are introduced to the diary of Tom Riddle who was a student at Hogwarts the last time the Chamber of Secrets had been opened 50 years before the time during which the book takes place. Through the diary Riddle shows Harry how he captured the culprit and during this memory we discover that Dumbledore was already a teacher and already quite old. Since we don’t know exactly how old Dumbledore was, let’s say he had to be at least 18 to be a teacher and 50 years later he would be 68. We know Dumbledore was already older so when the second book takes place he was well over 70 and possibly older than 80. I don’t have any grandparents, but I know some people that age at my church and I can’t imagine any of them doing anything that Dumbledore did.
Also, from the second book, we discover that Tom Riddle becomes Voldemort and when he created the diary he was 16 years old. The episode in the Chamber takes place 50 years later making Voldemort about 66. Harry is 12 in book 2 meaning that Voldemort had tried to kill him 11 years earlier when he was roughly 55 years of age. I think this is the part that bothered me the most because my parents are 58 and 59 years old and I cannot picture either of them running around with a wand trying to kill babies.
This does make me think of the Disney movie The Sword in the Stone, with Merlin being ancient and Arthur being quite young, so maybe all experienced witches and wizards are supposed to be old. However, Merlin was from a different time period and his story isn’t being told as if he were alive today whereas the story of Harry and the others is only about 10 years older than the present time (Nearly-Headless Nick says he died on October 31, 1492, and celebrates his 500th death-day in book 2 which would mean it took place in about 1992 causing the series to end in roughly 1998). Was Rowling trying to play off of a Merlin theme with the character of Dumbledore or was it merely a coincidence that the two characters seem so alike? And why did she make the villain of the story so old?
Perhaps I’m over-reacting or perhaps I don’t understand this because I’m not a witch, but once I started thinking about this I couldn’t stop. It was very intriguing and a lot of fun attempting to figure out the ages of different characters at the time they performed different tasks. This just goes to show that you can learn something new every time you read a book, even if you have reread it many, many times.