Parashas Beshalach · Passuk 13:21
Chabad.org — English translation + Rashi
Torah Temimah points to Bava Kama 92b (verified).
English: Rava said to Rabbah bar Mari: From where is that popular saying “When we were young — for the men; now that we are old — for the children”? He replied: At first it is written “And Hashem went before them… to give them light,” and later it is written “Behold I send an angel before you to guard you on the way.”
In context: See the cited source for the full section.
Derash on the verse.
The change from Hashem’s direct leading to the sending of an angel parallels the common observation that the strong once carried the weak, and later the reverse occurs.