No-one's ever too old to play Test cricket - the only reason age ever factors in is because sometimes (more often than not indeed) players' performances take a downturn into the second half of their 30s.
With McGain you can't really even tell that, as he's barely played before the age of 35. Quite clearly, he's the best spinner in Australia at the moment, and even if Casson were to mop-up the West Indies tail today (or tomorrow) that won't change the fact he's mostly been woeful and should never have gone on the tour, never mind played this Test. Currently ranks alongside Nathan Hauritz, Daniel Cullen (only against Bangladesh so not real Test cricket), Brad Williams, Gavin Robertson, Simon Cook, Shaun Young, Peter McIntyre, Brendon Julian, Wayne Phillips (the younger one) and Greg Campbell in the "luckiest players to represent Australia in the last 19 years" stakes.
That he's the best spinner in Australia at the moment doesn't mean he should be playing, obviously, and personally I'd reckon Noffke a far better bet, even in India. But rare is the team that tours India without a specialist spinner, even if their options are poor.