Knowing who you are traveling with makes a huge difference in my opinion about your question, but I have one more: where are you staying? If you are at a monorail resort, or an Epcot resort and will be using the International Gateway, that can make a difference in the answer to your question.
With a 14-year-old, I would absolutely do morning EMH at least a few times on your trip, and especially at MK. The amount of attractions you can get done in even that first hour will shock you. Also, if you chose to start tin Tomorrowland, for example, you could make a counter-clockwise circle, and from 8-9 get Buzz, Space, then keep your circle going and do a couple of choices in Fantasyland, Teacups, Pooh, maybe 7DMT, Peter Pan, all in that first hour or so, then continue the circle by being at the Tangled bathrooms very close to the rope drop (only Tommorland and Fantasyland are open for EMH, there is an internal rope drop at the bridge to Adventureland, Fronteirland, and before Haunted Mansion in Fantasyland when rope drop happens for all guests at the front gate). Near 9:00 or just after, having already ridden 6 or so rides, walk to Haunted Mansion, then make your way through Liberty Square and hit up BTMRR and Splash. With this tour plan, you are likely to get through all of those things by 10:30, having to wait in very, very few lines. At about 11:00, you'll see the crowds start to thicken. I like to make our fast passes for around this time, or head into Adventureland for Carpets, Jungle Cruise a Dole Whip, and as is tradition...Enchanted Tiki Room. We head out of the park by about 1:00 when it is the most crowded, and either eat lunch or head back to the resort for some swimming and a break. After our break, we hop back out most often to a different park to spend the evening.
This is just an example of a touring plan that has worked well for us in the past. We don't always do this, so,STIMES we spend the entire AM EMH in Tomorrowlamd then headed across the hub to Adventureland, or all of our early time in Fantasyland and headed right to Fronteirland, but the concept is the same. A lot can be accomplished in those first couple of hours. I don't think that can be understated, especially since although I'm sure you've read WDW is huge, with DL as your reference, I don't think you can appreciate the size or the crowds until you are actually experiencing them. You can go to Josh's site and see examples of a rope drop/EMH touring plan that he actually vidoes and you can see how much he gets done like this.
With all of that being said, I think being in the parks late also has it's advantages. We are advocates of Park Hoppers, because as previous posters mentioned, the early EMH park can sometimes be more crowded (because people without Hoppers start there and can't switch). Avoiding crowds at EMH parks is really a recommendation for the people that can't or won't take advantage of them. With Hoppers, just go to a different place in the evening. Nighttime hours are also fabulous...rides have lesser and lesser lines as time gets later and much can be accomplished in the evenings as well.
Additionally, our touring group is me, my husband, and our 11-year-old. For a week at a time, she has no problem getting up for EMH, touring until 1:00, taking a few hours to swim and relax, then heading out in the evenings until late, and repeating the process the next day. We do take a couple of "break days" where we spend the early part at TL or BB and don't get up as early on those days, then go to Epcot in the evening for Pm EMH to eat dinner, see Illuminations, and spend time walking out. And we'lol also have another "break day" where we may sleep in, spend some time at DS, then go to MK in the evening for Celebrate the Magic, Wishes, and evening EMH. Are we tired at the end of our 8-day-trips? Yep. But we do fine during the week with our breaks and we can sleep when we are at home. I don't want to waste any of my time at the World sleeping.
