I am into cat 4 now.
Cat 1 was around 2.5 at our first Christmas and never bothered with the tree at all.
Cat 2 again was around 2.5 at her first Christmas with us, she was climbing the tree (artificial) before DH had assembled it. Basically she spent the 4 weeks it was up in the tree. When we left her alone in the house we shut her out of the lounge. As she wasn’t very big she never managed to topple it over.
Cats 3&4 came next and were around 6 months at Christmas, whilst they were fascinated by the tree they didn’t really climb it. Although any baubles are considered theirs and we often find that the nicely decorated tree is naked at the bottom and over filled at the top as each time we put a bauble back in it goes higher up the tree.
We only have cat 4 now and as a mature lady she will just sleep under the tree.
Tips from over the years.
Put heirloom, glass and delicate decorations up high
Don’t Use tinsel or the hanging strands
No chocolates on the tree, my DC got the blame when they disappeared one Christmas, until I found them under the sofa
Try to limit access to the trunk, I found that if they can’t start to climb from the bottom then they don’t. I tend to use a wicker tree skirt and fake presents.