It is wrong to make assumptions based on physical attributes of the individual. Take for example the sexist idea "Men make better leaders" or "women are better multi-taskers" there is no way to know for sure that these things are true simply because of someone's physical appearance. To say otherwise is to promote differences that may or may not exist and therefore promoting stereotypes.
Discrimination is wrong. It does not matter if you are discriminating because of race, religion, gender, national origin, or age. Discrimination is still discrimination. People should only be judged on their own individual merits. If you judge someone based on any other factor it is not a fair or just analysis.
Ask yourself first: Why are individuals under 18 being discriminated? Then, ask yourself: Are all individuals under 18 being discriminated for that reason(s)? Next, ask yourself: Are those being discriminated being dealt with rightfully so? Finally, ask yourself: Is it a matter of logic and reason rather than morality? Because morality is defined as: beliefs about what is right behavior and what is wrong behavior ;the degree to which something is right and good : the moral goodness or badness of something. A situation regarding a moral issue and an adolescent where the adolescent is being discriminated may be hiring a person who is older simply because it is the norm, even though that older person may be less honest, lazier, or even less mature than the younger applicant. However, the same situation reversed where the younger applicant is lazier and less honest and the older applicant is the opposite is the exact reason as to why hiring people 18 years or older is preferred. Simply asking is discrimination against individuals under 18 a moral issue is like asking, "Is there a moral issue in discriminating against individuals with minor offenses on their permanent record?" Of course there can be an issue, but is it wrong of the discriminators to be cautious? And to those whom dislike younger people inherently, are they really discriminating against them? Or are they protecting their own interests by proceeding with their own business eschew of younger peoples interventions? These, and many other variables decide whether such discrimination is truly a moral issue.