I badly damaged my back when I gave my ds a piggyback at age 3. Had tingling in my feet & hands for 8 weeks almost constantly and off & on in my feet is still an ongoing issue 2 years later. Whoops! I knew I had a bad back & I shouldn't have carried him. After that I'd pretty rarely carry him and only for short distances (eg. a few steps if he got injured/etc).
I had to stop completely when he was 4 (and about 19kg) and I was pregnant. I was about 8 weeks pregnant when we went for a walk to see a road construction crew working in our street and after 20 minutes he refused to go home. I was sick with my pregnancy and I had to get home, but he kicked up a huge stink. I carried him home. I couldn't move for 2 days. That was it, no more lifting. Thankfully I didn't do any more serious damage that time!