"You are an old farmer."
Translation:'S e tuathanach sean a th' annaibh.
They are identical in meaning. In my dialect I would always use seann but other people would always use sean. If they don't accept seann, report it but you didn't say that was the case.
But to clarify for other people, seann always goes before the noun, and sean after or predicatively (when you say someone is old).
And no adjective that goes before a noun has a comparative. So when you learn to say 'older/oldest' you will need to use sean.