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."