There are pros and cons to a youngster in outdoor board. I will list both, then give my opinion at the bottom
Freedom of movement
If there are other hoses, they learn horse dominance, which makes them much nicer to work with people
They get to play
The air quality is better
If the pasture is not flat, they learn to pick up thier feet (ask any cowboy about a horse who has been raised in a flat pen and how fast they get sold when asked to do real work)
If there is grass, they can eat a greater variety of foods, which is healthier
They gain more coordination, and lots more muscle
It gives you, the owner, a reason to go out and do somethign with your horse. You can pull her out and feed her vitamins and any other suppliments she might need - which sets her up for being easy to catch for the rest of her life.
If she is in with bossy horses, she can get beat up and picked on
You have to watch her nutriton and weight. You can't just check on her once a month, you have to get out there at least twice a week to make sure all is well.
In cold weather, she may need a blanket, which the other horses might destroy
You don't know exactly what she is gettign to eat.
In my opinion, I ALWAYS keep yearlings and weanlings out in pasture. They need to learn to be horses. They need the muscle development they get from getting to run all the time. The only time I say no to this, is if she is getting beat up. I also use stalls so that I can monitor feed for hoses that I can't get out there every day to feed.