Supposedly that effect continues, the first third of life feels longer than the last two subjectively (at least I've heard that claimed). Wouldn't surprise me if it's a mixture of processing speed slowing down making things feel like they're moving faster and just the fact that each year is a small percentage of your life. Maybe also less variety day to day as an adult, fewer big changes.

You can test the clock speed thing and there's some evidence for it (ask different aged people to estimate when a minute has passed, stuff like that).