Uncle Tom's exact age is not given, but we do find out he is eight years older than his owner at the beginning of the novel, Mr. Shelby, who appears to be in his early thirties at that point, given the age of his son George. This would make Uncle Tom about 40.
Tom leaves behind a wife and young children when he is sold to settle Mr. Shelby's debts. Whatever his age, he is strong and robust. The narrator describes him as
a large, broad-chested, powerfully-made man ... There was something about his whole air self-respecting and dignified.
What is significant about this description is Uncle Tom's physical strength and self respect. This is quite different from the caricature of Tom that emerged in stage adaptations and other renditions of the novel after the Civil War. In these, Uncle Tom is depicted as a stooped, graying old man with a very servile attitude towards whites.
The real Tom is morally good but never servile. Stowe makes it clear that he hates being a slave and wishes for nothing more fervently than his freedom. He obeys his masters honorably because he decides to do so out of regard for his own moral compass. He is forceful both in body and soul and chooses death rather than moral compromise.
The later version of Uncle Tom is less threatening and was easier for whites to ridicule.