5 to 13 years old is the best age for Disney World. Older kids and the young at heart should be taken to Universal Studios, one of the many other parks and attractions in Orlando or the more adult portions of Walt Disney World. Some younger kids get freaked out by the Disney Character meet and greets. Not to mention waiting in those lines in the Florida heat.
If you have younger ones and you are going primarily for them then study the park and park rides and have a plan for your entire trip. You know your children better than an advertiser does. What few rides will they like the most and might actually benefit them? Know the approximate wait times and have a plan kids get bored and miserable on hot days and not all the rides will they be the perfect age for. Disney's Animal Kingdom is an example of a ride for most kids and adults even in the Florida heat.
The best ages for thrill rides? Although height restrictions apply to some of the rides this is because of safety and liability. What practical age and interest level does the child have? You may be best suited riding some thrilling rides or scarier rides yourself or not at all unless you visit with a more mature crowd or just you favorite person or people in your age group.
Disney World tickets are expensive even for children's tickets. If you take your kids to Disney World focus on them. Later, when they are at the perfect age or big kids, get some park tickets and they will remember their previous visit, return with you and have a blast on the actual thrill rides and at your own pace.
So many parents bring younger kids to Disney World and forget they brought kids. Most they don't want to go to the haunted mansion or the new star wars ride, roller coasters or the Kali river rapids to enjoy Disney. Focus on them and cherish the time doing things your kids love. later they will be at a good age to introduce them to parts of Walt Disney World that are worth adult prices.