DY: my observation makes sense because only about 10 percent of the guys – the cream of the crop – actually have the upper hand. Another 25 percent of boys are “mazeldik” or more realistic and get engaged fairly quickly. The average 50 percent struggle a little, 12 percent are stragglers to one degree or another and the remaining 3 percent or so are “stuck” hopefully not indefinitely. Working boys especially have a very hard time. My numbers are just “guesstimates”, totally unscientific.
There is no actual age gap as far as I know, just individual circumstances dictate when one will be married. I can’t say it holds true for all chasidim, there are probably age differences among groups. Generally speaking parents of boys want to see them married off fairly quickly as the nisyonos nowadays are great. Girls are a bit “safer” but it also depends on where she lives and on the girl herself. So if the girl can bide her time, she can be more selective as well.