The downside: In royal years, Charles is a teenager.
The main reason why the royals didn't use to live as long has to do with the following:
1. A lot of Usurpers of the Throne. Meaning a lot of Monarchs were killed before or killed during their reign.
2. Medicine was s**t. The average lifespan has increased.
I don't see Prince Chuckles reign lasting as long as his mother. I could be wrong though.