"Too young to walk" means "so young that he/she cannot walk". We use "very young" when we are not comparing the person's age with the ability to walk, talk or anything else.
So very just means "to a high degree or amount", but too means "higher than a certain degree, or more than a certain amount".
A wall can be very low, for example; but at the same time it can be too high for a small child to jump over.
1. This plate is very hot, but Mr Holland can hold it. 2. Mr Holland cannot hold this plate because it is too hot for him.
1. Was Mr Holland fat or thin when he was young?
2. Was he fat, or thin, when he was forty?
3. Who told him to get a bicycle?
4. Was it hard for him to learn to ride a bicycle again?
5. When did he practise?
6. Did he enjoy riding his bicycle?
7. Why did he like narrow roads?
8. Why could he pass cars at red lights?
9. Had the police taken Mr Holland's driving licence away?
10. Had they taken the other man's away?