I thought 'oes' meant 'yes'?
"means" is the wrong word, I think.
Remember that Welsh doesn't generally use a single word that means exactly "yes" or "no"; instead, it repeats the verb.
- Wyt ti'n barod? - Ydw. "Are you ready? - Yes." (Literally: "Am.")
- Oes gen ti siocled? - Oes. "Do you have any chocolate? - Yes." (Literally: "Is with you chocolate? - Is.")
- Ga i fynd i'r sinema? - Na chei. "May I go to the cinema? - No." (Literally: "Not may.")
- Ydy e'n hapus? - Nac ydy. "Is he happy? - No." (Literally: "Not is.")
So oes as a verb means "there is" when talking about existence of something. It doesn't "mean 'yes'" but it's sometimes used where an English speaker would use "yes".
But here, we have the noun oes, which means "age, period" (as in Oes yr Iâ "the Ice Age"); am oes doesn't translate to "for life" word for word but that's the equivalent expression in English (we don't say "a bag for an age").