When someone dies, the best person to ask about their age is gone. Often children do not know the exact age of their parents - friends are even less likely to know - especially if nobody had the date of birth written down because the couldn't write. It's easy to forget that. Ages at death on hedstones and certificates are often just a 'best guess'.
There should hopefully be a census entry from 1851 (if you can find him) where at least the guess will be from Thomas.
Roger