J.J. turned 17 months today. A milestone, because now, I can stop telling his age in months. From now through January, if anyone asks, he's "a year and a half". Then, through April, it will be "almost two".
I can remember when he was really little, and age was so much more specific... first days, then weeks (or partial weeks; "he'll be three weeks on Monday" or "he was 5 weeks yesterday").
If you ask Joe, he can give you J.J.'s age in days. Any day. He marks the total number of days each Monday on our white board calendar in the kitchen and apparently memorizes each week and adds or subtracts the days on demand. I'm hoping he'll give up by the time J.J. is in high school...
Because by then, I'll be saying "Oh, you know, he's a teenager...".