I think we have a pretty decent section on his age
Bender was originally constructed in 2998, making him 1-2 years old when Fry met him in 2999. The events in "Roswell that Ends Well" make his head 1055 years older than the rest of his body. The incident in "Teenage Mutant Leela's Hurdles" does largely not count as an ageing or the opposite, and fans largely regard his age to remain intact. In Bender's Big Score, Bender travels back in time several times, though the amount of times is unknown, the amount of Benders that appear at the end, may suggest at least a couple of thousand times. Given how many of these travels seems to be between 1000 to 5000 years in the past (from about 3007), an educated guess would make him at least a million years old, but probably more. An alternative theory could suggest when the time duplicates explode at the end of the first film, Bender is the same age he was at the end of the original run plus 995 years, as the Bender that went back 995 years in time to kill Fry is presumed to be the orginal and therefore wouldn't expire like the rest of them.
In the comic, "Rust in Peace", it is revealed that Bender, due to too much time travelling, must be inserted into a renewed unit. Farnsworth determines his age to be 27 203 before moving him to his new body. But 27 203 seems a lot less than what would be a quick educated guess. 27 203 would require Bender to have gone at least about a thousand years back each time and only gone 27 times, or instead gone more years back and done fewer than 27 times. But regardless of the ordeal, the amount of Benders appearing at the end of Bender's Big Score seems to contradict this fact. The event in the comic is thus not necessarily considered canon.
Speaking of liquid, in the commentary for BBS Ken Keeler mentions that the fact that metal is a liquid (over very long time frames) is a plot point in the fourth movie (ItWGY) but in ItWGY that point never came up. Presumably it was dropped prior to the story being finalised. Does anyone know (or has anyone asked) what that metal = liquid thing was supposed to be about?
It was assumed to be a joke from Ken Keeler's side.