Childs age??

Rachel • I have 2 amazingly beautiful boys! One turned 3 in December, and his little brother was born October 7th! 💙💙
How do you calculate your child's age? I have been going by weeks....he turns 20 weeks (which to me that's 5 months) tomorrow but the 7th was 4 months since he's born. I never really gave it that much thought but I'm totally confused....