I agree with the posters who spoke about letting kids be kids and evolve during high school.
My son has five favorite extracurricular commitments that have really mattered to him and in which he has done amazing work during his senior year. Only three of those were for four or more years. Of the other two, one he began in November of junior year, and one in August of senior year.
They were new interests for him, but they became huge interests to which he has devoted hours and hours of time. And he is showing leadership and having huge success with both activities. Both were mentioned in his application, but his big achievements with each came AFTER he had already been admitted early decision to his top choice college!
He was not hurt by having dropped some activities he did not enjoy as much as he had hoped when he started them (he did not mention those on his application) nor by having taken up some new interests later in his life (which he did mention). He will be going to a college that accepted only 12% of its applicants this year.
It would have been a serious mistake to deny him these pleasurable activities which also developed new interests and abilities for him. One even introduced a possibility for a future career.
As parents, we thought activities should be done for fun and a release from the pressure of all the hard work he was doing. We did not interfere with what he chose to do. We knew the activities would be part of a resume someday, but we did not encourage him to make decisions about them with a resume in mind.
But, when August of senior year arrives and the Common App opens, it is still important to take what a teenager values and has accomplished and present it to admissions officers in an appealing way. I think my son presented himself very well, taking what otherwise could be seen as very ordinary activities and using them to show how extraordinary he is.