Wow - some really judgemental comments here - obviously a very emotive subject. With regard the age when starting school, all four of my children were born in July or August and the eldest three started the January before they were five and the youngest started when she was 4 years and two weeks old (August birthday), whole year now start in September after 4.
With regards leaving children, by the time my eldest child, DD1, was at school, she had two younger siblings. When replying to an invitation, I usually checked whether it was ok to drop and go , which is what I did if possible. When I organised birthday parties for my children, I always made sure I had family/friends on hand to stay and help me.
By the time children have started school, they are used to being left. They normally knew who the birthday child's parents well and, if not, there were a couple of mums I knew quite well who would NEVER leave their child so I knew there was someone my child could go to in a emergency. I have to say, being quite strict with my children, I think it did them good to go to a party without me watching them the whole time.
There is quite an age gap between my third and fourth child (7 years) but, again, have left her at parties from school age. The only time I would stay at a party would be if it was either I was asked to help, if it was too far to leave and and then return or, in one case, it was a swimming party for reception children. The birthday child had two parents in the water, but also had a much younger daughter with them and I didn't feel happy to leave my daughter in this situation (2 adults with about 10 reception age children plus their younger DD in a junior pool deep enough for them to swim in and a splash pool next to it, as well as the main pool the other side). Admittedly I didn't go in the water but sat poolside so that I could watch my daughter to make sure she was safe.