A German study looked at professional violinists and found out that before the age of 21, they had all practised an instrument for 10 000 hours. Given that the violin playing started at the age of 6 mostly, it makes an average of 666 hours a year. In round figures we can say that these people are practising an instrument on average 2 hours a day for 15 years. Given that children have occasionally also other things to do (for example, they go to school after all!), lessons and exercises will also be missed, then clearly they will have much more hours in those days.
An acquisition of classical guitar playing technique is also concidered to take for 16 years and many of the curriculums are structured so. In principle the High School and University make 16 years. From here practising continues as not to lose technique. Only the technique is not what is needed, but musical development as well.
But when you play the musical instrument only for your pleasure, and do not necessarily want to make money with that on stages, you acquire basic skills quite quickly. This is what makes the guitar for a unique instrument as it is also a very simple way to make great music.