Kids graduating from high school that go directly into college are generally right on target, getting into the right places at an early time in their life. One of the main disadvantages of starting as an older adult is that minus the experience, you're a bit behind -- at least academically. Of course this shouldn't be used as a deterrent as you can easily catch up. The main downside within the classroom will be becoming familiar with material that you may not have studied, if you ever saw it at all, in years from your previous education.
Arguably the hardest part about being an older adult in school isn't the classroom at all: it's balancing the outside responsibilities. Many adult students also work full-time as they must support themselves or their family. Only a select few get to go to school full-time or work part-time jobs on the side. Family obligations can mean a person might have to work, go to school, as well as spend time with family. This can become exhausting if you're not up for the challenge.
Being older also means several advantages are brought to the table. Older students tend to be much more studious than younger adults. This partially stems from experience and maturity, but it also likely occurs because older students give up so much and risk a lot to go back to college, as opposed to younger adults who are often still under parental supervision and who know they have second chances later.
A career in place might mean less time while in school, but it can also mean rewards after graduation. As opposed to young adults who are only beginning their careers post-graduation, some older adults have established careers, where an education will only cause them to rise much higher in the ranks. Even for older adults that are starting new careers, they will then be armed with not only an education, but all the years of experience from any previous jobs.