Everyone at my child’s nursery wear tzitzit, except my son. The reason being that in the past only after bar mitzvah did men wear this. Children do not appreciate the significance of the mitzvah and at 4 I don’t think they should. Why has this changed nowadays?
The custom many have, of wearing tzitzit starting from Bar Mitzva or marriage is regarding the talit gadol, the large talit worn over the clothing during davening (prayer). On the other hand, regarding tzitzit worn under the clothing, which is what you’re referring to, the mitzvah is to regulate children from a young age, as all mitzvot, from the age that they can fulfill it, which is usually before the age of 4. Even if, for whatever reason, your son isn’t as ready as the other children, I definitely don’t suggest having him feel different and singled out, which may be particularly embarrassing at a young age. With Love of Israel, Rav Ari Shvat