Rabbit's Weight

Hello I'm new here. I took my rabbit to the vet for the first time today for his vaccination, where the vet also weighed him. He said that his weight was fine at the moment but he doesn't want to be any less than that. He is also going to be 6 years old this year (although we've only had him for 8 months). He also said that as rabbits get older one of the things to look out for is his weight/eating and now I'm just worried he's going to get really thin! He said his teeth appear fine and he does eat (pellets, veg and hay). Is this something that other people have experienced?

I'm no expert, but both of my rabbits tend to change condition quite a lot. They have both been a bit underweight over the winter but are now putting it back on quite quickly.

They don't seem to keep their weight on when it is cold and damp weather, but they seem to pick up when the weather is dry. Generally they usually look quite good.

They are 14 though ....