Rather rapid weight gain can certainly be a signal that senior citizenship has arrived for your pet. However, since weight gain is probably the first indication of seniority that most busy pet owners tend to notice, this can be a blessing instead of a curse. For example, it may cause you to make an appointment for a checkup for your pet.
Why another checkup, especially if you have been getting regular checkups for your pet annually? One reason is that a sudden weight gain that occurred after the last checkup can be significant. Another reason is that there are so many common geriatric diseases which are not obvious at a younger age that a checkup which focuses on potential dangers for an aging pet can mean a lot in terms of early identification of treatable conditions.
A senior exam can detect common old age conditions in addition to obesity such as cataracts, incontinence, cardiovascular disease, prostate or dental disease, in addition to a variety of less common conditions such as glaucoma, hypothyroidism, and anemia. All of these can be treated to prolong a healthy good life for years to come.
Incidentally, obesity is a problem with cats in many cases, in spite of the myth that they seldom or never acquire extra pounds. This is certainly true about many older cats.
It is pretty much common knowledge that during the last two or three decades, preventive veterinary medicine has come a long way. One solid reason for this fact of pet life - which contributes to the longer spans we are now witnessing in both coats and dogs - is our ability to confidently recommend a wide variety of specially formulated pet foods for senior cats and dogs as well as for younger pets.

