What is the best type of milk fat content for a normal 18-month old? Regular whole milk, 2%, 1% or fat-free (skim)?
It is recommended for children under 2 to NOT have reduced fat milk. The risk being that the calories lost by giving them reduced fat milk may not allow for adequate growth . Although "Overweight toddlers can switch to low-fat milk even earlier though, after their first birthday" .
From that evidence definately do not go to reduced fat until 12 months. After 12 months, only do it if they are overweight, otherwise you may affect their growth.
After 24 months, I would do it at your discretion. If your family drinks reduced fat milk, it is easier for them to drink it as well.
Our Doctor suggested using toddler milk up to 24 months and then regular milk.
If you skip toddler milk, regular milk should be used at least up to 24 months (see this reference), unless they are at risk of being overweight.
Also, we were warned to always add sugar when using cow's milk. (Unfortunately I cannot find an online reference for this.)
My baby is dairy-allergic, so we give her coconut milk that is certified dairy-free. I'm not sure if I'm allowed to mention the brand, but you can google it and it's pretty easy to find the brand; it comes in shelf-stable boxes as well as refrigerated cartons.
There are significant benefits to coconut milk because the fats are medium-chain fats (as opposed to long-chain fats) and contain fats that are present in human breastmilk. This study (PDF) details some of the health benefits and how the blanket recommendations to avoid coconut milk because it was dangerous for the heart were unfounded.
The silver lining to my daughter's dairy allergy is that we discovered something far healthier for her than whole milk that we never would have even thought of otherwise.