I have three children, and one received a very delayed DTaP at the age of 5 (He had gotten a nail embedded in his heel and his pedi thought it really necessary to prevent tetanus).
I did not notice any reaction to the shot.
A year later, my oldest (age 8) fell and scraped his head on the metal handle of a dresser, which cut into his scalp. It was at night, and the ER physician again thought taking a solitary tetanus was very prudent, which I agreed to. They gave him the adult dose, single vax, and although he had injection area soreness for over a month, he had no other noticeable after effects. The ER Doctor told me that the adult dosage was actually LESS than they give to children, because adults need less to stimulate the immune response? I've never independently verified that, but if that is true, that would really make the case for waiting, in my opinion.
I will share the experience my friend has had, with her delayed vaccinations. She delayed and her son was being pressured into being caught up before he started kindergarten, and because homeschooling wasn't an option she went ahead and let him get several vaccines at once. He immediately developed a fever, head & speech tic (within 24 hours) and several other disturbing psychological symptoms. (zoning out, focusing issues, violence & anger & screaming). Months later he hasn't recovered, and they are testing him for possible reactive pediatric MS, because now white matter is showing in a MRI they did. His tics have never gone away, and although they are very noticeable, her doctor refused to report it as an adverse reaction, he stated 'they must not have noticed them before'. It's a heartbreaking situation.