I am old, so perhaps my theory may be relevant. For an old person, it is less easy than it used to be to do anything physical, such as household chores, odd jobs, etc. An old person therefore moves more slowly than they used to when they were young. This means that the said household chores, etc. take longer to do for an old person. The old person tends not to notice too much that they are doing things slower than they did, but all of a sudden they realise that their list of daily activities is nowhere near being completed by the end of the day. The old person exclaims, "Where does the time go?" It's a logical, convenient explanation of why things don't get done as quickly as the old person expects. It's not the person who is going slower, it's the time that's going faster.