You have put your finger on the important distinction, and it is the key to the whole thing: this is about memory, not about experience.
Ask somebody how fast this morning is going and age makes little difference. Ask how fast the last decade went and it makes an enormous one. So whatever is happening, it is happening in retrospect.
The leading account is about how densely a period is encoded. Your brain does not store a continuous recording; it stores distinguishable events. When you look back, the length of a period is judged largely by how much retrievable material is in it.
A year at ten contains an enormous amount of first-time experience - new school, new skills, new places, everything unfamiliar and therefore encoded in detail. A year at forty, with a stable job, a familiar route and a routine, contains far fewer distinguishable events. Same duration, far less to retrieve, so it reads as shorter.
That also explains the lurches you noticed, which the ratio theory cannot. A year with a move, a new job or a child in it feels long in retrospect, at any age. A year in which nothing changed compresses to almost nothing, and that is why the effect is not smooth.