weeks vs months and years?

Tatiana🇨🇷♈️💍🇩🇴♏️ • Happily Married. 💏 👰🏽 12.19.08 💙 07.10.16 💓 02.29.20 💙 🐸 🤱🏽BreastMILK is best 🚫 formula
My husband asked me this question the other day and I had no idea how to answer it lol.. 
he asked me why when you ask women how old their child is they respond with he/she is 26 weeks or 15 months instead of just saying 6 months or 1 year old?