My Eliza is nearly 6.5 years old now. She had an eye removed in September where her weight dropped slightly but she managed to regain and hovers around the 900g mark.
Eliza used to weigh around 1150g at her heaviest when she was in her prime a has steadily dropped some weight since the age of about 4.5 years.
If you are concerned about anything, it may be worth a vet visit just for a general check up.
During the course of last week, Eliza suddenly became very boney around her back and shoulders despite eating like a horse and Sod's law, I had no working scales.
So, we went to the vet on Monday where she had a full check over including her teeth and poos. Turns out she is perfectly healthy and hasn't lost any weight but in fact gained 30g since her last weigh in.
Eliza has plain porridge oats daily, baby corn / corn on the cob a few times a week, Timothy hay with dandelion and marigold (fed in moderation), barn dried grass and I have just ordered some alfalfa hay again to feed in moderation on top of the meadow hay I use for bedding which they also eat.
I have no idea if any of this helps to maintain her weight but I like to think it is helping.