My hamster Harty

[HartyHetty]

Hi all, I have 2 syrian hamsters (different cages!) but recently the older one Harty (17 months) is no longer coming out of his house as much, both hamsters get out of their cages and put in their hamster run ball every night roughly 9pm til they are tired and no longer run then they get put back in, Harty for the last couple weeks, maybe a month, is coming out occasionally through the day for food and water but then straight back into his house is this normal in older hamsters or should i be worried??