We found 12 dictionaries that define the word age groups:

General (7 matching dictionaries)

1. age-groups: Merriam-Webster
2. age groups: Collins English Dictionary
3. age groups: Vocabulary.com
4. age-groups: Wordnik
5. age groups: Cambridge Advanced Learner's Dictionary
6. age groups: Wiktionary
7. age groups: TheFreeDictionary.com

Business (1 matching dictionary)

8. age groups: Legal dictionary

Computing (1 matching dictionary)

9. age groups: Encyclopedia

Medicine (3 matching dictionaries)

10. Medical Dictionary (No longer online)
11. online medical dictionary (No longer online)
12. age groups: Medical dictionary

(Note: See age_group as well.)

Definitions from Wiktionary (age group) ▸ noun:  A demographic grouping based on age. ▸ Also see age_group Phrases: Age groups in Scouting and Guiding, more... ▸ Words similar to age groups ▸ Usage examples for age groups ▸ Idioms related to age groups ▸ Wikipedia articles (New!) ▸ Words that often appear near age groups ▸ Rhymes of age groups ▸ Invented words related to age groups