I have been involved with pedigree assignment since the seventies, and have struggled with the problems with both age systems. Over the years, I have developed my own age system that is a compromise between the two, and has a twist of its own.
I have always favored the aging system. Aging allows a breeding program to progress from foundation horses for generations; your sire that was foaled in 1979 and was considered 'young' when you first showed him in 1985 is now and aged sire, with several generations of offspring out there breeding. I like aging because of the possibility of endless generations recording great model horses -- just as real horse pedigrees record the history of great real horses.
But I also despair having to 'kill off' my old show horses (some of who still show very well!). I like to think that I choose names and pedigrees carefully, and after years it's hard to develop a new identity for a model, especially a unique horse heavily campaigned in the show ring. He will always be that particular identity to me, and to those familiar with him. I was also bothered by the fact that the same 1979 stallion, now 18 years old, doesn't look his age. The same is true for dapple greys. Most vivid dappling (the best color for the show ring) occurs when a horse is relatively young -- too young to be really useful on an s/d list.
There was also no provision in either system for what showers sometimes call 'historical models' -- those models created to represent breeds, types, or colors that no longer exist in present day horses. Probably the most obvious example of a historical model is the Quagga (which became extinct in the 19th century), but there are also rat-tailed Appaloosas from the 1960s, colorful Spanish Horses from the 1600s, and similar types. As much as I enjoy historical models, there wasn't any way, with the existing systems, to add historical models to my sire/dam list.
So I began using my own system which I call Era Specific Aging, which changed how I saw time and aging as it related to models. With ESA, I assign an appropriate foaling date for the model, without regard to the horse being 'alive' in the present day. In other words, my model done as a portrait of a 17th century Spanish Horse could be assigned a foaling date of 1635. He is done from a painting of a horse from that time, and would be far more likely to exist as he is in that time than in this one, when Andalusian-type horses are grey. Now, of course he would not be alive today, nor still producing foals some 362 years later! So I also assigned him a lifespan -- say, 20 years. That gives me 1636-55. Of course, he couldn't possibly be bred to a horse in this day and age; he could only have been bred to mares that were alive and old enough to produce foals during the years of his breeding career. He would need to be bred to a mare of his own 'era,' just has his foals and future descendants would belong to their own eras. Although it's unlikely to breed enough generations this far forward, with other individuals (say, a strain extinct since 1930?), it would be quite possible to breed lost lines and colors forward to present day breeding stock. And the nice thing about Era Specific Aging horses is that they are completely compatible with ordinary aging horses because all horses are Era Specific. It's just that most models on sire/dams lists are specific to this era!
But of course, there is still the problem of these models with far distant foaling dates (or even more recent ones) not looking their 'age.'. My Spanish Horse surely doesn't look 362 years old! He doesn't even look the 20 years old that he supposedly 'lived.' So I have assigned him the age of 10 years, because he looks that age. For showing purposes, he is ten years. The way I look at it, the model is an image of the identity I created, just as a photograph would be. And just as breeder can use a photo of his stallion at a younger age when advertising him, and label it "Famous Stud, pictured at age 10." I can do the same when I show, I am showing and promoting their image at specific age (probably when they looked their best), but the identity will age, or if it's from the past, it had a lifespan. And I can continue to show the image (images, like models, never die, even if the horse represented does).
I hope that others find this system useful for them.