The Bible isn’t clear on the exact age of those Jesus chose as His disciples, but this professor is engaging with speculation rather than fact. I believe Jesus chose His disciples regardless of their age. Rabbis generally established schools to which students could come and learn from a chosen teacher. Jesus, however, selected His disciples personally.
Some have suggested that Peter was older, because when Jesus was criticized for not paying the temple tax in Matthew 17, he told Peter to go fishing and then take the coin found in the fish to pay the tax for both Jesus and himself. These people infer that Peter and Jesus were the only ones in the group over the age of twenty and therefore the only ones required to pay taxes according to the Roman law. Others infer the disciples were younger because Jesus called them little children in John 13:33. If they were adults, that term would have been insulting to them. It seems more consistent with Scripture to say that He called them children because they had so much to learn. And the truth is we all have a lot to learn, no matter how old we are.