Three thoughts:
1. I like the idea that as they age they grow Large, but (as things are now) that doesn't make it a viable PC race. Perhaps suggest that as characters retire, they set down roots (heh), their movement becomes zero, and they grow Large and are very content with where they are.
2. For the natural AC, are you suggesting a flat AC 13, or 13+dex? Other races allow the dex bonus, but that obviously makes this more powerful. Giving a flat 14, with no dex bonus (they are slow and ponderous) but they can use a shield, would also work.
3. Flurry of spines is too powerful. I'd suggest: when the cactoi is not wearing armor, with a bonus action can release spines so that everyone within a radius of 10' makes a dex save doing 1d10 piercing (2d10 at level 11). Once per long rest.