Time
Years
noun
Years old in months shows time in years. Just imagine counting time by months and years... You kind of do anyways, right? How old are you? 5 years old. Or, 5 and a half years old (that's five years six months). So, it makes some sense.
さいげつ
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
それから半世紀の歳月が流れ、彼のことなんてほとんど忘れかけていた時に、私達は小さなパン屋さんで偶然再会したんです。
After fifty years had passed and I was almost starting to forgot about him, we ran into each other in a tiny bakery.