Do you guys have the 05 update, as I think it changed the slower movements to 55. At least all my games are 55. I just checked to be certain, and a set of parents are 52 & 51 and are walking very fast when their energy is higher.
I agree they slow down too much at 55 though.
They also live longer with the 05 update. I had one live until 70, but the other set of parents were in their late 60's. I didn't know exactly as they went during the night, but they were over 65. I guess that's still young, but if they hang around too long we'll be saying "come on already, I want some new family members so I can have kids & activity again".

If you do have the 05 update, are your games original ones before the 05 update come out?