On average years ago, I found my rats lived to 26 months. Nowadays I find that 22-24 months is more the average, with tumours, and other things showing up much much earlier with so much bad breeding going on.
My youngest adult rat to leave me was 10 months, but she was very sickly, but her sisters made it to 15 months (abdominal masses) and 25 months. They all were born at a shelter.
My oldest rat was Sebastian, a rat I got as a rehome at 22 months. He ended up with a lot of physical problems/conditions but no real illnesses at all. My beloved brave tiny man stayed until 38 months old when a small tumour in his throat was impeding his breathing and I let him and his beloved sickly girlfriend go, Seb's slightly older cagemate, Pippens, was pts at 37 months old and Pip's brother was 32 months when he contracted pneumonia but he was in terrible shape when I took in the 4 boys.
I find if you have a rat that makes it to 22 months then just enjoy every day as much as possible afterwards.
I have only a few oldies here.
Moth - 28 months old, she is just starting to slow down slightly. She's in great shape!
Hestia - 25 months old, is showing her age the most physically, with 2 small tumours, and a rapidly advancing spinal nerve degenerations. Happy and brighteyed though and somehow this girl can still climb! 8O
Shadow - 26 months old, took her in as a rehome in May. She's chubby but active and happy. Slight wheeze occasionally but its not from an active infection but scarring of the lungs.
Bella and Lisbet - 26 months old - new girls I was contacted about from my local HS. They wanted them to go to a good home for the remainder of their lives. They are doublerex and that is very uncommon here and could attract the "Oooh I want! Look it has no hair! That's freaky and different!" adopter.

A bit of respiratory we are working on, but active and healthy otherwise.
Enjoy those Golden Months people!!
When some of our rats slow down and like to cuddle and be loved on more than they used to. Its bittersweet but its part of the lifecycle, and remember that rats do not care about quantity so much as quality of life. They do not care if they only lived 18 months as long as they had a great 18 months. :mrgreen: