At what point do you stop using weeks to describe your LOs age? I think stop at 12 weeks and start saying 3 months etc. what do y'all think?

Ma
My LO is 13 weeks today but I'll just say 3 months old. I've seen some say "my baby is 40 weeks old." 
Yes but your baby is 9.5 months old. 
🤷🏼‍♀️