The Gedolei Hador have previously exhorted the Tzibbur and Bochurim to make every effort to marry at the earliest appropriate age. Therefore, we encourage Bnei Torah and their parents, if it is consistent with the advice of their Rabbeim, to consider shidduchim at age 21. May a sweet and pleasant life be bestowed upon those who listen, and may they merit to build upstanding homes in Klal Yisrael, and to raise children and grand-children involved in Torah and Mitzvos.
Even that is a strange message. Why did they print that, and not age 19? Why not age 12? If they are really just saying as early as you think is appropriate, why peg it?
Also, in what world do rebbeim decide when a boy is ready to get married? I’ve never met any rebbi who would think that is his position. I’ve met boys who thought their rebbeim would, but they were wrong.