"Han skriver sin ålder."

Translation: He writes his age.

[PotatoSanta]

Is 'sin' supposed to be pronounced with a 'sh' when spoken fast? because that's what I am hearing.

[Arnauti]

sin should normally be pronunced that way when it appears after a word ending in an r. We usually assimilate r + s over word borders too. (some dialects have special R:s and don't do this, but Standard Swedish does).

[Koguze]

Is it correct to say "hans" instead of "sin"?
Han skriver hans ålder

[Yerrick]

Yes, but the meaning is different: your translation would mean he writes some other guy's name, whereas "sin" means it is his own name that he writes. English is actually ambiguous in this case, so either could be a valid translation from the English.

[makmegs]

What does this sentence mean? He writes like his own age, he writes his age (on something)?

[rhblake]

He writes his own age, perhaps on a piece of paper.

[ManolisDel1]

Det finns åld också ... So im thinking about åld- er

[ManolisDel1]

gammal också