Readings & Meanings:
childish, very young
Although he's sixty, because of his babyface features he's often mistaken for 30.
When children turn 4, they go to kindergarten.
Instead of 糸, we have Ito-gashira, 幺. It's still a thread. In this case, 幺 lends a pronunciation "ヨウ" to 幼.
The other element is, of course, 力, power.
How much power does an infant have? As little as a very thin thread.