Courses can be completed online, with the caveat that one semester must be spent at the physical college (most people just rent an apartment over the summer to finish up). Most undergraduate courses there cost less than $1000, so a full 32 course undergraduate degree costs considerably less than one year of school in many prestigious traditional 4 year colleges. And once completed, your son or daughter will be able to say they have a degree from Harvard (it doesn't matter that they used the extension school, the degree will be the same as if they enrolled in the usual way).
Classes are offered on both weekdays and weeknights, so it's easy to schedule school around work. Many courses are available online, and for the undergraduate degree, only 16 credits (4 courses) are required to be taken on campus. All courses are transferable, so if your teenager just wanted to take a few courses for college credit and have them transferred to a local college, you could do that, too.
The classes will be more difficult than the typical undergraduate fair at other colleges, but that's not surprising since, well, it is Harvard. To be honest, most college courses are much less rigorous than a lot of the homeschooling high school curricula I've seen people using, anyway.
I think what we're planning to do is start with trying one of the basic courses in the fall of the ninth grade year; if it's too advanced, we'll know the specifics of the course difficulty and will delay it according to our child's ability. If the child excels, we'll build our "high school" curriculum around taking two to four college classes per semester (focusing on the courses they're taking and reserving homeschooling study of other subjects for the summer months). In theory, they could graduate at the age of 18. That's probably too optimistic, but at the very least they will have earned some college credit. :)