Gerbils have much shorter life spans than we do. This can be disheartening at times because we so quickly lose them from our lives. Entering an older age can be a hard transition for gerbils. In the wild most die prior to becoming elderly. But when they're domesticated they have no predators and can live much longer. But they also need special care as they get older to continue to live happy lives.
Keep warm: As gerbils grow older their immune system grows weaker. They often have issues keeping themselves warm (some will even lose patches of fur). A quick way to keep your gerbil comfortable is to attach a small terrarium heater to a bottom corner of your gerbil's habitat. Hermit crab heaters work wonderful for this purpose. The heater will create a warm area for your gerbil to cuddle away when they get cold.
Offer fresh water conveniently: With old age using a water bottle can become a tough feat. So instead use a very shallow bowl (tupperware lids work great for this- so do reptile water dishes). This allows very easy access to water where they can effectively hydrate without having to drink drop by drop.
Make food easily accessible: Just as with water, food should be very easily accessible. Keep food in a large dish in an easy-to-access part of the cage. Even two or three dishes makes sure your gerbil won't have to move far to fill up.
Massage stomach for digestion: If your gerbil is not using the bathroom regularly, you can lightly massage their stomach with a q-tip to induce proper digestion of food. Lightly doing this 15 to 20 minutes after a meal is a great way to help your elderly gerbil digest their food.
Ask vet for supplements & advice: Small pet veterinarians will often be able to check up your gerbil and give you advice on making them more comfortable in their elderly state. They can also offer supplements and nutritional bits that will help keep your gerbil healthy.
Keep cage familiar but occasionally add new items: As gerbil's get older they're less active, but that doesn't mean they shouldn't be engaged. For the most part keep your gerbil's habitat relatively the same. But occasionally add a new item or two to keep them entertained. This will mentally stimulate them without causing stress over an over-whelming habitat change.