In spite of physical appearance and the occasional adult-like, verbal profundity that springs forth, teenagers are kids. Kids have several "jobs": to study, to learn, and to grow as individuals. They also should learn responsibility, accountability, integrity, and common sense. For many this will come easily, but for some it will be more difficult. Regardless, all teenagers come to the point where they "think" they are all grown up and should be allowed to make their own decisions. One of these decisions is often to find a paying job. This is not done merely for the sake of wishing to be grown up, but for the practical reason of wanting/needing income. For most teens, however, it comes down to "wanting" versus "needing".
Many would argue that teenagers can learn the aforementioned character traits by holding down a part-time job. Thinking parents, however, realize that teens can learn these very same things by staying in school and using their free time to improve their academic record. These same parents also realize that the teen years are a period of transition, and that teens still need assistance with envisioning the future and developing the best plan for that future. They also know that perhaps 95% of a teenager's "needs" are truly only "wants".
A teenager wants extra cash to buy clothes, or CDs, or makeup, or, heaven forbid, drugs and alcohol. A teenager with a job may also need to pay for a car or insurance to get to and from that job; but if he didn't have a job, he wouldn't necessarily need that car. Most parents do make every effort, some going above and beyond the call of duty, to provide for their kids' needs. Some encourage part-time work to allow the teen to provide their own wants. This can be a mistake.
In some jurisdictions, teenagers can legally work at age 14; in most places, the younger the teen, the more restrictions are placed on time of day, total number of hours, and the type of work to be performed. A majority of teens do not begin working until age 16 when most obtain their drivers licenses - for the simple reason that parents may balk at ferrying their teen back and forth at all hours.
Obviously there are some positive points to a teenager having a job: he will have his own money and will not be asking his parents to finance things like prom, movies with the guys, dates, and so forth; and he will quickly learn the value of a clock, and that 4:00 p.m. does not mean 4:45 or 5:00 p.m. Hopefully he already has or will learn the latter from merely attending all his classes each day. As for the former, parents are allowed to say "no", or to say "yes" with restrictions: simply because a teenager asks politely, doesn't mean he always should get what he wants. On the other hand, there is not yet a corporate entity called "The Bank of Mom and Dad" - and some parents may prefer that a teen earn his own money by working for someone else, and not be paid through that fictional financial institution.
Why, then, should a teenager not hold down a part-time job? As previously stated, studying and school are both priorities and will provide a sound basis for his future, whatever he decides to pursue later in life. Family is important, both for values and for service as part of a unit larger than self. He may be involved in school- or church-related activities, and parents and teens will have to decide which endeavors are most important. The crux of all this is "time". Every teen has the same twenty-four days as everyone else, and teens, more than adults even, need a proper amount of rest as well. All kids, regardless of age, tend to be overscheduled and hassled and stressed already. Is it really in your teenager's best interest to add more?
Think, too, about this extra income - where will it go? Some will/should/might be saved, especially for college, a car, or whatever is decided by both teen and parents. Some will undoubtedly be spent on clothing or accessories that the teen feels are so very necessary but that parents are unwilling to finance. A lot of will just be spent. Of course, teaching a teen to manage money is a very important and worthwhile job, and most schools do not have this in their curriculum - it's up to the parents.
Illicit activity. Sounds scary, right? Many teens who work not only have more disposable income, but more time which can be unaccounted for. Many parents do not thoroughly check on their kids, job or not, and if a teen says he has to be at work at a certain times, some parents fail to check this; they assume that since their teen is "old enough" to work, he is old enough to tell the truth and be responsible. This isn't always the case. Teenagers hang out at the mall; if your teen works at a mall, he may be up to who-knows-what either before or after he's scheduled to work. And he has extra cash. Parents need to be aware, as always when dealing with teens, that what is hoped for isn't necessarily what is actually true.
Younger teens, especially, can get caught up with older teens or adult workers in many kinds of "trouble" - drugs, alcohol, theft, vandalism. The influence of older employees on younger teens cannot be discounted. If your teen works twenty hours per week, that's twenty fewer hours that he is under parental influence - these ages can be very susceptible to negative suggestions and pressure, and even though it is a period of transition, make sure your teen is well-equipped to resist temptation. Many, still, cannot handle the decision-making and personal integrity required in the "real world".
On the one hand, you have a teenager who is begging for employment, who can certainly put his paycheck to good use, and who could also have some life lessons emphasized as well as apply good time and money management skills. On the other hand, this same teenager may not be equipped to responsibly handle the increase in both his own time and his own money and who may be quite vulnerable to the affects of unknown types of peer pressure and influence.
Let your teenager be a kid, for just a while longer. He has the rest of his life to be in the workforce, and you probably need the extra time to prepare him for the next stage of his life, college or tech school or apprenticeship or a career. Don't forget that while teenage years are rather transitional, he is still a kid.