My parents separated when I was 18 (and on a gap year in Russia at the time in the days before internet / email and little contact with them while I was there). My sister was 16 and living at home. They divorced a few years later.
Personally, and speaking just for myself, I think how it was handled had the biggest effect on me, rather than the separation per se. Basically, I was extremely upset at the time and over the last decade or so I have continued to be upset by my dad's insensitivity.
Bad way to handle it:
My mum has to ring me (in Russia) to tell me that dad was visiting "friends" in Canada and he and she were splitting for a bit. Didn't realise he had any friends in Canada and mum refused to tell me who they were. Turns out it was the OW.
Better way to handle it:
Dad calls me himself and takes responsibility. Parents are much more honest with me about what was going on (I only gradually found out that there was an OW). My mum shouldn't have tried to spin some story.
Bad way to handle it:
Dad basically never tells me what is going on with the OW, or that he is leaving mum for her, or that he is going to marry the OW(yes really! Only found out when he sent me an invite 10 days before the wedding, when it was hastily arranged in the UK rather than Canada because they couldn't get the paperwork done for Canada). Only meet the OW, by this time my father's wife, at my paternal granny's 90th birthday, where neither my father nor the OW bother to introduce her so I spend the first 15 mins of the "party" wondering who my new step mother is.
Better way to handle it:
Dad thinks about what impact his marrying OW would have on me and my sister. Dad thinks about how best to introduce her to us.
These are just a few of lots of examples of how I think the whole split was badly handled in relation to me and my sister, and we were relatively old at 18 and 16! The split has affected my sister much worse than me and she and my dad are only just about on speaking terms over a decade later. It's created problems at events like my wedding, my DD's big 1st birthday party, because part of the preparation for both was "how to handle OW and my mum potentially being in the same room".
So I think my messages are:
The split / divorce in itself needn't have had the negative effect it had / has on me and my sister. It was the way which my parents, in particular my dad (the instigator of the split) handled it.
My dad assumed that my sister and I would be able to deal with it much better because we were older teenagers. Not the case IME, again because he handled the whole thing so rubbishly. Basically I think / know he thought it was just something between my mum and me and wouldn't really affect me and my sister because we had almost left home. Completely untrue.
My dad put hardly any thought into how the split would affect us, his children, and how he should behave as a parent to us. Big mistake to assume that just because your children are older they don't need parents.
I should finish by saying that I think both of my parents are happier without each other, and I really don't know why they got married in the first place or why they were attracted to each other, so completely objectively, they are better off apart.
I hope this helps.