ideally, yes you should be able to tell them anything.
but we all know that people sometimes don't take criticism very well no matter how nicely put.
and if they've also noticed the same things that you mention, they may have already have had fears and now you've just confirmed them. it is a very delicate area for most people. people like to believe that love (and attraction) will be strong and not wane with the changes that happen over time. it is foolish to hope against what we know happens, but most of us are still fools.

I think when you truly love someone, the usual change of looks due to time and life are expected. So would I love someone less because he got a little flabby around the middle or got a few wrinkles? No. I'd love the flab and wrinkles. If he had an accident and lost his legs or was terribly scarred in a fire, yes, I'd still love him--perhaps more due to his strength to survive.

But if he let himself go due to laziness, became enormous or developed poor hygiene, we'd have a loooong talk and work to correct it.