1/8 or 17cm doll (what age?)

1-2 year old normally.

 

I think it depends on the doll and its proportions and also whether you prefer to think inside the box or outside of it.

Yes some are maybe designed to represent a young child but others aren’t as clear.
Those with more fantasy or exaggerated aspects could be any age really. Small doesn’t always mean baby or child, from a factual point of view there are conditions such as dwarfism for example, then there’s also various otherworldly beings of lore such as the fae folk who come in many shapes and sizes or what about fictional races such as Tolkien’s Hobbits?!
It’s down to the imagination of the individual and how they perceive distinctions of age and size.

 

A doll that is 17cm tall and in scale 1/8 would represent a full scale person who is 136cm tall. So an older child, perhaps around 10 years old, or so.
But that assumes a realistically proportioned doll.

If the doll is proportioned as a smaller child, the scale given is probably off. If the doll is proportioned to represent a two year old at 17cm, that would make it closer to 1/5 scale.

You can look up charts for average height for different ages online and then you just do the math.

But, as Blodeuwedd points out, it depends a lot of what you are going for style wise. Very often dolls have exaggerated proportions with enlarged heads, and very small hands and feet. Sometimes they are made to be cartoon-ish and you can apply almost any age you like to them.
Very often, the context you place the doll in is what will determine how you read the doll. The scale of furniture and props is a big one, especially common place things that usually have a standard size in real life. The size and look of other dolls is another big one. If you place a Narsha next to a JID, she reads as an older child or pre-teen. If you place her next to a 55cm doll with a similar head size, she reads as a toddler.