Pasture board is not only a more affordable option, it is a healthier one. Horses are meant to roam, graze and breathe fresh air.
Sadly most boarding barns do not have proper ventilation in the barn and the horses have to breathe in dust, dirt, possible mould all day/night.
All my horses have been/are on pasture board (on grass during summer and hay during the winter months) and they are happy and healthy, only negative is my mare is a EASY keeper and usually needs to be put in a paddock (fat pen) for a month or two during the summer.
They don't get bored (learn to crib, weave, stall walk) they get natural exercise.
Horses are meant to roam so why not let them
only negative is some days it's a long walk to catch if your horses doesn't come when called
Best of luck, age should not be a problem as long as your boarding barn has a pasture with horses that will not beat up on your baby