I don't do it by age, so I'm no help there. Depends on the weather when they are born, one year it was around 105 when two were born, and they had thick foal coats, so we did put them in our lap and clip their chest, neck and bellies at around two weeks old.
I usually clip the moms neck, face with foal in attendence, then put the foal in my daughters lap to clip the head and neck, but if the weather is really hot I may clip more. I do like my foals born early as we like to show, so if they are born the end of Feb to April, I don't feel the need to clip early as the coat is protective, especially with the cool nights. They usually don't get a full body clip until they are weaned or if the weather is extremely hot like it was in the year stated above. Before I bath ans clip the full body I like them to be halter broke and taught to tie, but, instead of tieing them the first time, I have someone hold them for a warm water bath so if they struggle there is some wiggle room, I put them in a grass area to dry in the sun, then get someone to hold for what I call a rough clip. Then the next week or two we repeat the process only this time, when clipping I put them in the cross ties, which by this time they are trained for, and give them a really good clipping. Most of my guys when started young can have their ears clipped without a twitch, that is my goal. I've bought some mature hoses that must be clamped or twitched for ears & legs, it becomes a pain. Patience and reward works so much better.