If GP meant "there's no difference", they're certainly wrong; a four-year-old is obviously 33% more advanced than a 3-year-old. But the question here was about

If GP meant "there's no difference", they're certainly wrong; a four-year-old is obviously 33% more advanced than a 3-year-old. But the question here was about socialization -- can a 3-year-old and a 4-year-old satisfy one another's needs for peer socialization? Is a 4-year-old who socializes with 3-year-olds and 5-year-olds but no other 4-year-olds being socially deprived somehow? I think the answer there is clearly "yes" and "no", respectively.