Personally I'd like to see Japan's WNT play the U-15 Boys. I'd be willing to bet $100 that it would be a very tight game.
It is indeed difficult in general for women to play against boys who have already hit puberty, essentially young men. That said, I think the other problem is that our USWNT for the last 25 years has dominated the competition primarily because of athleticism. That's their trump card. But when they play boys that trump card is completely nullified, so it's not surprising they struggle against older boys youth teams. They're not going to outrun them to the ball or win high crosses in the air.
I'm pretty sure all the women NTs scrimmage vs. top u14/u15 boys teams. Maybe a poster who can do a search in Japanese could find the results for you.
I don't watch much women's soccer but I do watch women's World Cup and pay attention to the build up to the women's World Cup. About ten years ago I remember reading women's teams from all over the world had trouble scoring, let alone beating, even 7th grade boys teams. The big problem for the women was that they were used to shooting high soft shots over the GK, because most women GK at the time were short and could not jump, but those shots were easy saves for the boy GK, who was both taller and could jump higher.
The women's game has improved a lot since then. The GK are better, taller, jump higher and the women have better shots and can shoot a greater variety of shots. Nowadays I read reports of women NT scrimmaging 14 and 15 year olds so I'm guessing that is now the cutoff age for giving the women NT the challenge they need.
But countries like Japan have clearly demonstrated that women can easily surpass the technical ability of our boys youth teams. On the men's side during Spain's 6 year domination of world soccer, they changed the game by showing how physically less-opposing but highly technical players could just completely lock-out the opposition with total ball domination regardless of the size and athleticism of the opponent. It doesn't matter how big or fast you are, if you're only getting 20-30% possession, there's only so much you can do.
If our USWNT embraces a more possession oriented style of play and recruit highly technical players to play in that style, I'm sure they would be more competitive against boy youth teams and more importantly they can stay competitive in the women's world scene, which is rapidly improving.
Japan has fully embraced Coerver training and they are producing a bumper crop of highly technical players, both men and women. If you look up "Japan" and "coerver" on YouTube it is amazing watching their youth drills. I've tried to get my son to do more Coerver touches on his own, telling him you aren't playing 4 hrs. a day in a Brazilian favela, get your touches more efficiently with Coerver. He will only do so much and coaches have told me kids will not do a lot of Coerver drilling.
I think it's easy to go around blaming the coaches, a pay to play system, emphasis on winning, blaming kickball because it's easy to coach and win with at younger age groups, emphasis on size and speed because it's an easy way to win. But I also blame the kids. They can cut half hour of PlayStation time and go to the park and do Coerver drills, or cones, or wall ball or shoot at a net. Even 15 min. a day in the driveway doing Coerver touches would help a lot. But very, very few kids do it.