Two things. First, a lot of young people took part in the playtest. Enough to have a good sample size, so the ones that didn't playtest it don't mean anything, since they were represented.
Sure
But from Wizard's own words the majority of current players of 5E did not play dungeons & dragons before 5e.
Those younger players during the play tests were players of 3E and 4E.
Second, 5.5e didn't change drastically enough to change the target audience, so either it's aimed at older players like 5e was, or the original was aimed at young folks with some nods to the older folk
Because it couldn't change.
5e was not allowed to change when it became 5.5 because the fan base had the
sunken cost of buying books made for 5e and they did not want to lose those books.
The fan base
demanded that the books that they bought from Kobold Press or Ghostfire Gaming or Magehand Press or Darrington press or whoever they bought books from, all the PDFs that they bought, and a homebrew that they got from Kickstarter or whatever they found it.. they
demanded them to be backwards compatible.
That's why Jeremy Crawford would not shut up about it being backwards compatible.
The fans wanted things changed but
not as much as they wanted the books the spent money on to be still useful.
I mean people couldn't get around their heads that some classes start at level 3 and old subclasses just get there first and second level features at level 3 now.
It's the latter. 5e's base structure is aimed at young people, not older players. But they put in some nods to older folks to try and bring us back to the fold. Those nods do not aim the edition at us. They simply drew us in to the edition aimed at the younger folk.
Nah bro
5e was aimed at grognards
That's why the 2014 dungeons Masters guide did not have any tools on teaching you how to be a DM and mostly had conversion rules to convert your old game to 5e and wasted entire chapter on the planes and minor planer gimmicks.
5E was an attempt to snag back the OSR crowd but make a game that they always are crowd could get there younger siblings children and nieces and nephews to play with them.
It didn't work They always are crowd stayed with their OSR games. The 3E crowd went and played Pathfinder 1 and Pathfinder 2.
And the younger kids mostly played 5e with the 4e crowd who no longer had the digital tools to continue playing 4e but could not find a 13th Age group big enough to play.
I know from personal experience that if it was lame, it was a DM problem. My 12+ games were very enjoyable, as were the ones I played in as a player
It is a DM's problem 5e high level play is boring to most of the 5e playerbase.
It takes a exceptionally good DM to make 5e high level interesting to most of the 5e player base.
That's the point 5e was made for
people who are on the older side.
5E was literally made to grab 1E, 2E, and 3E fans.
If you were 25 when 3E came out,
you are 50 now.