In the historical Jewish culture, it is recorded that many youth were graduating from formal schooling by the age of 15. At that point they would go on to become apprentices for different trades (i.e. blacksmith, fisherman, farmer). This is good to remember because Peter, James, and John were all fishermen when Jesus called them to follow Him (Luke 5:1-11).
Another point which we cannot overlook is the fact that Jesus was 30 years of age when he started his ministry assignment on the earth. As it was Jewish custom, Rabbis usually took on "disciples" or followers to train at the exact age of 30. Jesus then calls these disciples "little ones" multiple times in the scripture (Luke 10:21, Luke 17:2...)
Now in Matthew 17, something significant happens. It was time to pay temple taxes, and since the disciples had been traveling with Jesus, they had no way of paying. Jesus then tells Peter to go grab a fish and look in it's mouth and you will find a gold coin - take it and pay taxes for both he and Peter. (The Greek word used in verse 27 was stater, which was the exact amount to pay temple tax for two.) But why only those 2? According to Exodus 30:14-15, only those who were over the age of 20 had to pay temple tax. So that is conclusive evidence that Peter was possibly the only disciple over the age of 20. In Matthew 8:14-15 the scripture references Jesus coming to Peter's house to heal his mother-in-law, which means that he must have been married. (Peter was the only one of the disciples that we know was married during the time when Jesus was alive.)
Matthew was known to be a tax collector (Luke 5:27) which is usually reserved for those who might have been adults. However, there is no conclusive evidence on this so we can only speculate.
So what does all this mean? The disciples were following Jesus at the age of most high school students. At the time that we are usually sending students off to college, the disciples were launching the first church and adding thousands daily to the faith. So if they can be used at such a young age, with no formal theological teaching or credentials, why can't you be used in the same way? What's your excuse? What's holding you back from God using you in a mighty way?
Never let circumstances of this world keep you from the best of God. He's not looking for your ability, He's looking for your availability. All you have to do is say yes to God, just like the disciples.