noun
period of rapid growth; growth spurt
Refers to a stage of life, especially childhood or adolescence, when physical development is most pronounced.
彼は発育盛りで、毎日たくさん食べる。
He is in a period of rapid growth and eats a lot every day.
Compound of 発育 (hatsuiku, 'growth') and 盛り (zakari, 'peak' or 'prime').