The issue I often take with parenting is the word "should".
Many parents, I've noticed use the word the same way a comedian commented about drivers. As in "There are only 2 kinds of drivers on the road: Maniacs & Morons. Maniacs drive faster than you, and Morons drive slower." So "should" becomes whenever THEY either chose to, think they'll choose to, or their child did.
In reality SO many things enter into MOST aspects of childhood & parenting. Including, but not limited to,
- development (physiological -including neurological-, and psychological)
- personality
- situation
- custom / tradition
Some kids are more than ready to shower on their own and wash their hair to a T at age 3. Then at age 8, all of a sudden have hygeine issues. Other children aren't ready to bathe themselves until 5/8/10/whatever. Some kids regress, some don't. Some kids are big on personal space, some parents are big on personal space, some kids are super social and want the company, ditto parents, some kids are physically incapable, some kids are scared, some parents ditto, some kids take too long, some kids aren't thorough. And that's all just psych & personality. Not even entering into being physically capable (like arms that don't reach yet, or a disorder), or situational, like a kid who would be fine with short hair needs help with long hair. Much less custom & tradition, which often treats bathing as either the highest of sins sans sex, or bathing as family time. (Ditto cooking... there was a scene from a show a million years ago that had 2 families sharing a house for a week. One mum was spoon feeding her toddler and another toddler walked in, opened the fridge got a juice box & yogurt, opened each, threw away the garbage, and set them on the table. The first mum was stunned and said to the 2nd just walking into the kitchen: "He got his own SNACK???" the second mum looked at her for a second and said "No... he got a snack earlier, I asked him to grab one for me."
When "should" is is whenever it WORKS for your family. Sometimes kids will surprise you and be totally capable of X YEARS before you thought they would be, sometimes they're not years after you thought they would be.
It's whatever WORKS, in my not-so-humble opinion, and that changes in every house, and often for each individual within that house.