I don't have a lot of experience to speak of- not even my own since I've only been twice (recently) in my whole life!
But I can only speak about the experiences we have had and the ages of our children.
I think it all depends on the child, ofcourse- but for US...... this is what I'm thinking.
Our youngest was barely 2 the trip with the family and she spent the majority of the time in the stroller (except on the few rides she could ride and WOULD ride also). Probably not a whole lot of fun for her- but she didn't complain. And she was just a little too young to have the "light" in her eyes at seeing the characters/etc- she'd either get scared or shy.
Our middle child was barely turned 5 before that trip- just young enough to still be thrilled about everything, and amazed, etc- but old enough to walk with no complaining. The ONLY thing I can say is that he did not like being too short for some rides- but that probably wouldn't have been such an issue if he didn't have a big sister that COULD ride them. LOL
Our oldest had fun on the rides- but at almost 9yrs old (and I realize all children are different- I know I would have been different had I gone at 9yrs old!) she did not get into the characters/etc. And she CAN be a whiner occasionally- and didn't enjoy us all going at the pace/schedule of the younger two, especially the youngest.
Now, our next trip this November? I'm pretty sure my son will still be "into" the magic, our youngest should be even more (she is totally into the princesses and such already- and talking about living at WDW in Cinderella's castle ofcourse LOL) excited and enjoy herself even more at that age...... and my oldest? Well, we've already planned to try to make this trip more fun for her- and as it works out.... pleases my DH more also since it's not exactly his idea of the funnest thing in the world to do (ie: going nonstop at WDW and US/IOA. He's not big on "rides" really either. He's that way at Six Flags also and he thinks it's uncool for a grown man to get excited about WDW, Mickey/etc. *sigh*). What we plan on doing is having some days or afternoons where he goes back to the resort with the youngest, maybe the middle child too sometimes- and then we'll have some time where our oldest can ride what she wants to her hearts content (lines allowing LOL) and not have to ride "baby" rides and not have to go as slow as the youngest child/etc. I think she'll enjoy that a lot better. And heaven knows she'd enjoy a little one on one time with mom- because ofcourse the younger two do end up taking most of my attention most of the time (especially the youngest).
So hopefully- even though she'll be almost 10- she'll have a great time..... even better than last time!
I don't know what I'll do when they get to the teenager years- if they don't enjoy WDW as much as myself. I guess I'll just make solo trips..... party poopers. LOL