I only have my own experience and my childrens to draw from. My Father started us on 28gauges and it was a disaster. They were light, but we shot heavy loads through them and got kicked. I was so glad to move up to a 20 gauge.

My own kids were started on 5-1/2 to 6 pound 20 gauge SxS with 3/4oz loads. I purchased a spanish 20 gauge for cheap with a short stock, intending to cycle it through the kids.