Why is the predicative form of 'old' used attributively here? As far as I know it should be "An e seann iasgair a th' annaibh?" Having "sean" come after a noun like "iasgair" is only possible in a sentence like "tha an t-iasgair sean" 'the fisher is old' (= predicative use of the adjective), not when the adjective modifies the noun directly (as here).
sean (rather than seann) is being used attributively here. An e seann iasgair a th' annaibh? = An e iasgair sean a th' annaibh?. Seann and sean both mean "old" but are used differently. Seann precedes and where possible lenites the noun and is only used attributively (like the small number of other adjectives that precede the noun). Sean follows the noun when used attributively and can also be used as a predicative adjective - An e iasgair sean a th' annaibh? / a bheil an t-iasgair sean?
Is that an innovative feature of the youngest generation of Gaelic speakers then? It sounds ungrammatical to me to use sean attributively after the noun, even more so if I imagine it being used with a feminine noun: craobh !shean 'an old tree' (for seann chraobh). When I learned Gaelic I was taught that sean is one of the adjectives that precedes the noun (when used attributively ofc), and that it is then mostly (though not necessarily) spelled seann. I have never encountered a deviation from that rule before.
I don't think it's innovative (but I'm not a linguistic historian) to use sean attributively after the noun, but seann preceding the noun is more common. There is a study here of differences in usage/meaning in seann vs sean/aosta, as well as droch vs dona and deagh vs math https://www.researchgate.net/publication/332709084_The_conceptualising_function_of_Scottish_Gaelic_preposed_adjectives