Being Hubbards and grown for the table, you would normally be looking at 10 to 12 weeks old to slaughter them, depending on the size of bird you prefer. The growth rate is far quicker than utility birds, so it's recommend to introduce grower pellets at 3-4 weeks. My Hubbards are almost 4 weeks old and are starting growers today, mixed with chick crumbs for a few days.