It's not always easy to explain how one does things, but you did pretty well for a quickie request. However, there are some things I'd like to take note of since you did ask for a critique.

If you're going to make an aging tutorial, I'd be a touch more thorough. Going from baby to senility, taking gender into account, and even adding the variations of body types if your feeling adventurous, can really make an 'OK' tutorial 'Great' because it gives your reader a better source to draw from. I'm not saying you didn't work well with what you were given, but just something to keep in mind in the future.

I'll admit, I'm not partial to anime for it's rather generic body types/faces/etc, but even in anime there's a drastic change from birth to adulthood. Whatever your style or chosen look, proper anatomy and proportions are the building blocks of it all. Every peer, artist, and instructor that I've admired and asked for tips have always stressed this point.

While you did hit upon some of the highlights of how the face changes through the years, note that most changes in the face are due to not only growth, but where fat is distributed.

I do have to wonder where his jaw went by the end of it. I understand this might be a stylistic choice of yours, but even then it's still good to have a general idea of facial structure and where everything goes. Here it looks more like you grabbed his chin and pulled it down like taffy. Even here, where the chin is more prevalent in both the Diamond and Inverted Triangle face shape, you can catch a glimpse of the jaw.

Now, about your "facts." Yes the arms and legs do change in relation to the body, but here you might want to explain how. How long do they get per age? Why do teens and adolescents' limbs look disproportionate, etc. Ideally, the calves and thighs are nearly the same length as the torso. Your forearm--from wrist to elbow--is as long as your foot. Yes, anime tends to fudge these rules, but any good artist understands that getting down the basic anatomy is key. Even cartoonists understand anatomy.

If by detail you mean how fat has given away to muscle and skeletal definition, then yes. But that's not to say fat can't be detailed. Lines are a great tool for showing age and definition, but it's better to understand where to place these lines so as to achieve the look you're going for. Too round a line under the pecs, and your guy could have had man boobs. Sometimes fat can give older folks a rounder form--which is often associated with childlike or normally nonthreatening features. (Take Dr Facilier and compare him to the pudgy and less-than-threatening Lawrence. The same can be said about Jafar and Iago,
and though I apologize for sticking to only Disney villains here, I hope the formula of making the rounder one the less-threatening is obvious.) Other times it can sag--which is an effect of gravity over time on our features. It all boils down to how you want to portray your character.

Now, while I can't argue the exact ratio of head-to-body-height since varying sources have different definitions for how tall you should be normally or if you were a fashion model. For convenience, I'll list what my anatomy book has.
At birth (Age 0) - 4 heads
Infancy (2+) - 4 1/2
Childhood (8) - 6 1/2
Puberty (14) - 7
Youth/Maturity (18-40) - 7 1/2
While some of your listed numbers weren't too off, don't stick to eyeballing it since that can leave you with some wonky looking anatomy. Measurements are your FRIEND.

Be thorough with your information, as your readers are going to use these measurements as guidelines. You don't want to bog them down with too much information, but more like cliffnotes to keep in mind when they're drawing.