• ВПАДАТЬ/ВПАСТЬ В ДЕТСТВО (в МЛАДЕНЧЕСТВО)
[VP; subj: human]
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1. to lose one's mental faculties because of old age:
- X впадает в детство{{}}≈ X is entering (falling into) his second childhood;
- X is getting (becoming) senile;
- X is in his dotage.
♦ "Не все в старости впадают в детство", - сказала Тёща... (Зиновьев 2). "Not all old people fall into a second childhood," observed my mother-in-law (2a).
2. Also: ВПАДАТЬ/ВПАСТЬ В РЕБЯЧЕСТВО to act, think etc as a child would:
- X впадает в детство{{}}≈ X behaves like a (little) child (kid);
- [in limited contexts] X is experiencing (enjoying etc) his second childhood;
- X is being childish.
Большой русско-английский фразеологический словарь. — М.: ACT-ПРЕСС КНИГА. С.И. Лубенская. 2004.