"Your mothers are old."
Translation:Vos mères sont âgées.
Actually, there's nothing wrong with it.
une vieille dame
In fact, I'm not sure if "agees" means old or older...
Google Ngram seems to agree with you here: http://books.google.com/ngrams/graph?content=vieille+dame%2Cdame+%C3%A2g%C3%A9e%2C&year_start=1800&year_end=2000&corpus=19&smoothing=3&share=
"Tes mamans" isn't a good translation for "your mothers." "Maman" is what people call their mothers; in English, because there are so many possibilities, it has a lot of possible translations: Mom, Mum, Mommy, Mummy, Mam[m]a, Ma...
I might use "tes mamans" when talking to a child (I know a lot of kids who have two moms, so it's something I actually do use in both French and English) but in that case, the English would be "your moms" or "your mommies" (Heather a deux mamans) rather than "your mothers."