Not necessarily.
Depending upon the learning curve and exposure in the mix, a Force-user is likely to become more powerful over time but aging will eventually take its toll at some point. Although exceptionally learned Force-users are a force to be reckoned with even in old age.
Unless a Force-user have figured out a way to overcome negative implications of aging which is exceedingly difficult task - immortality and/or unnatural ways of prolonging corporeal life coupled with physical fitness.
I think it really depends on the potential of the Force users in question. However, *most* Force users we've seen(at least in canon) do seem to have become stronger with age. Maybe not stronger from a physical standpoint(depending on how old they are), but definitely stronger with the Force.
This fact is actually noted in the RoS novel:
Besides, there was something about growing old that made her connection with the Force even stronger. When the body began to fail, the mind reached out, unencumbered by physical ability.