Jesus did not begin his ministry until he was about thirty years of age. He died three years later.
Why didn't he begin before that? For one, you weren't really an adult until that age in the hebrew culture. It seems to me that it was the Father's will that Jesus was fully mature before he began.
In addition, there was some major preparation before Jesus's ministry. John the Baptist was an important part of God's plan, being the forerunner. He preached for years, preparing "the way of the Lord". He later baptized Jesus.
Why didn't the gospel at least tell us about His early life.
1. It was probably uneventful/ordinary.
2. Because of (1), it was probably not well-known to those who wrote the gospels.
Hope that is an okay answer.