婴儿、儿童、少年、成年 and other words related to age
chinese characters related to age, there're three main sources.

the first one is 論語, chapter 為政

子曰：「吾十有五而志于學，三十而立，四十而不惑，五十而知天命，六十而耳順，七十而從心所欲，不踰矩。

that 

而立 - 30+

不惑 - 40+

知天命 - 50+

耳順 - 60+

the second one is 釋名, which was written in 漢 dynasty

in 卷三﹒釋長幼 (page 57-60):



"人始生曰嬰兒﹒胸前曰嬰"　嬰 - newborn, before breast (breastfeeding)

"兒始能行曰孺"　孺 - started to walk

"七年曰悼"　悼 - 7+

"毀齒曰齔"　齔 - losing baby teeth ( 9-12?)



"十五曰童"　童 - 15+

"二十曰弱"　弱 - 20+

"三十曰壯"　壯 - 30+

"四十曰強"　強 - 40+

"五十曰艾"　艾 - 50+

"六十曰耆"　耆 - 60+

"七十曰耄"　耄 - 70+

"八十曰耋"　耋 - 80+

"九十曰鮐背", "或曰黃耇", "或曰胡耇", "或曰凍梨", "或曰齯"　鮐背, 黃耇, 胡耇, 凍梨, 齯 - 90+



"百年曰期頤"　期頤 - 100+

the third one is 說文解字, also of 漢 dynasty, which was published earlier than 釋名. some explanations were different from 釋名, e.g. 耄 is 90+.

btw, talking about age, remember to consider "虛歲", or "虛齡"

http://dict.revised.moe.edu.tw/cgi-bin/cbdic/gsweb.cgi?o=dcbdic&amp;searchid=Z00000111350

there're several more age related characters / words, have fun :)