“At what age do children's speech delays appear?”

A lisp is not a speech delay but rather a speech disorder. Delay implies that it just hasn't developed. In your son's case, I would have him evaluated by a speech pathologist that specializes in myofunctional therapy. Lisping is usually the result of positioning of the tongue in which air is escaping through the front of the mouth or the sides of the tongue. Myofunctional disorders can be the result of genetics, thumb or finger sucking, teeth position, bottles, pacifiers, Sipee cups, etc.

Speech delay can occur at any time including at birth. Speech means communication. So a newborn if he doesn’t interact with eye contact or shuns contact is delayed. What you are talking about is a lisp. It starts when a child starts talking. Lisping can be due to structural deviation in the mouth or the use of sippy cups or habits. It doesn’t go away by itself and your child will need speech therapy. Start when he is old entropy understand. But get an evaluation if there is anything that can be done by a dentist or orthodontist. At times if there is a tongue thrust problem speech there should be done before an orthodontist.

It depends on the child. They don't read the child development text book.

They can appear at any age after ~ 3 years old.