At what ages does a child fully understand the concept of past tense?

For my child development coursework I am looking at a 4 year old child who is an over achiever in most areas but says 'goed' instead of 'went'. I asked his father recently if this is still happened and he told me that it take a while for a child to gasp the concept but i was just wondering about about what age does this occur (on average), also any other details welcome :)