M.D.
We started putting our son in an exersaucer around 4 months old, because he was exhibiting a desire to do something with his legs. When we held him, standing on our lap or a table, he'd jump and jump and jump, which was surprisingly exhausting for us, and thats how we knew he was ready for the exersaucer. I think readiness is more to do with leg strength than the ability to hold up his head, but if your son likes it, go for it.