Hamster becoming less active

Hi, my Syrian hamster, Jemima, is 1.5 years old. She's recently stopped coming out as much and sleeping for longer, but other than that, seems fine. I can't help but think it's a bit early for her to be slowing down. Should I be worried?

Any advice much appreciated. Jemima is my third hamster so I am quite experienced, however my other two remained quite active as they got older.