Hi Kim, Gina asked that I take a look at this question. In my opinion, for a gerbil of that age, it is not safe for them to be flying.
Not only is the situation extremely stressful, but the altitude alone can make them severely ill. You know how your ears pop on an airline? That's from the high altitude and cabin pressure. With an animal that tiny, the affects will be much more severe, and I'm afraid that he make not make the entire flight.
Hamsters and gerbils are also notorious for having strokes as they get up in age. Being that yours is 2.5 years old, combined with an airline flight, puts him at great risk for that possibility.
If you can find any way around not taking the gerbil along, it would be the best option.
Hope this helps!
I am NOT, nor do I claim to be, a Veterinarian.
Always check with your vet for any symptoms your pet exhibits.
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