But where I board does not offer pasture board w/grain every day. Will this affect her growth?
Age is not an issue with being pastured. I've had them born in a stall paddock. Kept the mare and foal there for a couple of days just to make sure everything is fine with it's health, nursing, etc... and to handle the foal so it's use to me. Then they're both out to pasture pretty much for life. Come in for feedings only because I like them to be use to coming.
Mine are easy keepers and get no grain. Good grazing, and a mid quality hay since I don't limit their hay. They also get copra (coconut with about 10% of the oil remaining) as their "feed". It's a cooler feed and cuts down on the issue I find with feeding grain to my easy keepers.
You can always look at feeding her yourself. One of my daily pleasures is having my horses perk up and head to their feeder when they see me coming with the buckets.