Can and individual's teenage years be considered the "best years" of their life?
Of course these years can be considered the best of a person’s life. However, just because they can be does not mean that they should be.
There are certainly things to be said for one’s teen years. A teen has, in some ways, a good combination of freedom and lack of responsibility. Teens do not have full-time jobs. They do not have to provide for themselves. They do not have to care for children. This frees them up to have more fun than many adults are able to have. Teens also have more freedom than younger children. They are typically allowed to do more things because they are older. Finally, being a teen can give you a sense of possibilities. Things are exciting because you do not know what is in store for you.
However, it would really be sad if your teenage years were the best of your life. It would mean that you peaked with 60 or 70 years of life ahead of you. I would say that later years are better. You have a more settled life. You might well be married and thus have love and stability in your life. You may have children who love you and whom you love. You do not have the same sense of uncertainty and insecurity about who you are that many teens have.
Overall, then, I would say that the teen years are not the best of your life, even though some people might say that they are.
Yes, of course they could be. However I don't think a person would ever consider the teenage years the best years of their life until much later in life. As a teenager it is hard for most to appreciate not having the amount of responsibility an adult does. They are also going through changes physically and mentally making this a challenging time for teenagers. So I cannot ever imagine a teenager saying, "these are the best years of my life." However maybe later in life once the stress of work and having to survive in the real world is realized, then would someone look back and say, "Wow! I sure had it good, those were the best years."