It is (or was) on the official recommended leisure reading list for 7-10 year olds in England. More recent lists seem to put it for 11-12 year olds. ("Key Stage reading list watership down" is what I searched for, and sometimes it's stage 2, otherwise 3.)

I think I was 7 or 8 when my mother / English teacher gave it to me.

Somehow I'd more or less forgotten about it (even though I'd read it twice myself by age 9). So what we thinking, it'd be ok for a 6yo? How about a 4yo?

There is explicit violence, bloodshed, and death. Make your own judgement.

Haha, yeah it doesn't sound great. It's hard though with these things. Ideally you need to see a snippet of the sort of it to judge yourself.

I would just read it yourself, its quite an enjoyable read for adults.

The website http://www.arbookfind.co.uk/default.aspx is sometime helpful for finding "appropriate ages". Schools use the system to help children find books matched to their reading level.