It's normal, but we don't really know why people tend to sleep less as they get older. A lot of things we traditionally viewed as a normal part of aging are now seen as disease pathology. She's also been doing this for 16 years, so age doesn't really explain it anyway.

That is a common misconception: https://sleepfoundation.org/sleep-topics/aging-and-sleep

It's not that they need less sleep, it's that they don't sleep as well as they did in earlier years.

Sleeping is harder but you still need the sleep.

Well as someone over 45 I can say if I get less than 6 hours I'm functioning on half my normal IQ. Engineering requires deep concentration for me that means 8 hours sleep if I want to perform as aggressively as my juniors.