Sounds like dimdegau to me... if it was, by the way, would this be the correct word for the noughties?
Working on this word and also chwedegau brings to mind a question on a usage that needs both words (and happens to be a true statement for me). Would it be correct to say "Ges i fy ngeni yn y chwedegau, a dw i mewn fy mhumdegau"? If not, what would be correct? (For the avoidance of doubt, and please also tell me how to say this, I will be fifty-five next month.)
- Ges i fy ngeni yn y chwedegau. - Fine
Dw i yn fy mhumdegau. - A specific fifties (your, their, the, etc), so yn rather than mewn
Byddwch chi'n pum deg pum (mlwydd) oed fis nesaf., or
- Byddwch chi'n hanner can (mlwydd) oed fis nesaf. (another, older way of expressing fifty, and sometimes still used for age)
The blwydd/flwydd/mlwydd is often left out of ages, and we don't cover it on this course anyway - it is an older word for 'year of age' used only in that context nowadays.