1. They'll Likely Already Be Trained
This is one of the biggest advantages of adopting an older domestic animal: it's far, far more likely that they'll have had training, either from a previous owner or from the people managing their shelter. This includes everything from toilet training to tricks to responding to various cues.
One of the most complicated parts of having a young animal is the training process, and while any new animal in your house will need to have a new education (where things are, what your specific rules about scratching or play are, what your personal routine is), the foundation of knowledge an older animal brings with them can be a massive benefit. If they have bad habits, you'll have to work to replace them with better ones, but the fact that they'll already know what training is will likely be a massive help with that.
2. Their Personalities Have Developed
One of the wonders of owning a small animal is being uncertain about how they'll develop and what kind of grown dog/cat/rabbit/ocelot they'll grow into. However, if you need a bit more surety (a cat who won't scratch your kid, a dog who isn't prone to eating your shoes or his own poop), an elder animal is likely the way to go. Even if their background isn't known (some shelter animals come in as strays), they'll likely have more settled personalities, from the extroverted to the chilled-out, and that gives you something more definite to work with.
3. They're More In Need Of Rehoming
People don't think about senior animals when they think of new pets. It's a harsh truth, but it's accurate. The typical narrative about pets is that you get them from the beginning to the end of their lives, but there are many other ways in which the story can go: our cat, for instance, belonged to a woman who died tragically (and was left alone in the house with her for a while before her body was discovered). People sometimes need to give up animals because they're moving or can no longer care for them; pets sometimes run away and can't be returned because of a lack of ID. And so animals who could have been with one owner all their lives come back to the shelter to wait out the end of their lives for a new one.
4. Animals Live Longer Than You Think
"Senior" doesn't necessarily mean "about to pass away at any moment." It can, of course, but you'd be surprised at the tenacity of domestic animals, even those who've been through the mill. As a rule, pedigrees and purebreds die earlier, while crossbreeds, mutts and moggies have a more extended lifespan. My husband's mixed-breed tortoiseshell cats lived to 19 and 21; the average lifespan of a domestic cat is 15, but many things can affect that, from lifestyle (indoor vs outdoor) to diet.
With dogs, size is also a consideration, with the larger breeds tending to have shorter lifespans because of strain on the organs. Average lifespans across many dog breeds run firmly into the teens, with poodles expected to live until 14 and border collies until 13. And when it comes to animals like tortoises, you're likely in for several decades, if not longer. So don't think that adopting a senior pet means you'll only have a few brief years together.
5." Senior" Doesn't Automatically Mean "Requires Extra Care"
The twilight years of a pet's life may include more vet visits, but that doesn't automatically mean you're doomed to dealing with catastrophic illness and spiraling veterinary expenses. An elderly animal is perfectly capable of being a healthy animal for a good portion of its life, and it's now possible to get cheap insurance specifically for animals over seven or eight. (Yes, that's what counts as "senior", which is acceptable for a dog or cat expected to live until 10, but perhaps not quite realistic for the ones who'll comfortably hit the teens.) Be prepared, particularly if you have an animal with a particular special need, but you don't need to be unduly alarmed.
6. ... And If It Does, That's A Great Gift To Give
Older animals often do require specialist help; it would be false to pretend that isn't true. And ultimately, an elderly animal in your care is your responsibility, up to and including ensuring the comfort in which it passes away. This can be an extremely difficult burden, and I know that's a hard thing to take on; but that's the responsibility you bear with any life in your hands, including that puppy who one day will be old and grey at the muzzle. The alternative for senior animals left unclaimed is a death in a shelter or sanctuary, where staff do their best but can't offer the focus, care and attention of a loving home. (Some shelters also have high euthanasia rates for older animals.) Loving a senior pet becomes an exercise in selflessness. You'll be surprised what you'll do to help one.
7. You Can Be Part Of A Brilliant Community
People who rehome senior animals tend to find each other. There's a reason the Old Friends Senior Dog Sanctuary is incredibly popular on Facebook (1.5 million followers and counting, plus a serious Presidential campaign for the venerable Mildred the pug), or why the accountant who lives with 9 senior dogs and a pig called Bikini is an Instagram star. And then, of course, there's Corduroy, the world's oldest living cat (Guiness World Record-certified), who at 26 has been officially "senior" for 18 years. People who devote themselves to senior animals can get addicted for life; I'm not sure I'll ever go back to puppies or kittens. Plus, an elderly animal support network can be a lifeline when things go wrong (they're animals, so they will eventually get stuck in something).
If you're in the market for a pet, don't restrict your search to young animals, unless you have good reasons for specializing in them. Consider a senior pet. They may be the grannies of the species, but they've got a lot to give (some of it's slobber, but still).
Images: Unsplash, Giphy