Hmm - I feel a bit embarrassed about commenting on someone I know nothing about. I don't think she looks young or old for her age.
I tried to carry my son in a sling when he was about 3 months old. I found him too heavy even at that age and I just got too HOT. I was regretting the decision not to use the pushchair by the time I got to the tube and had a train journey and a mile long walk at the other end. He used to slobber all over the sling as well. I never used the sling again for him.
My daughter was much lighter, she didn't slobber and we had a different sling. It was much much better but I still used to get hot.