Sleep Apnea

Even though there is substantial variance about weight, age, genetics, phenotype, etc, when dealing with any one person's onset of sleep apnea (three types of sleep apnea), age does play a factor because, as we age, we gain weight and our health generally deteriorates due to aging mitochondria and other degradations too numerous to list here.
One degradation is kidney function, and that is as a result of the pituitary gland's lowering, again age related, of the production of ADH, or anti-diuretic hormone. When we're young, we don't need to get up to pee (unless on a bender or ill, or if we have kidney disease). As we age, and our ADH production drops, we have to void more, especially at night. This is probably the major reason why all older adults, whether apneic or not, have to awaken.
If your question is more intended to be, 'What is it about sleep apnea, or its treatment, that makes waking early a problem?', it again can be multiple things. Sleep in the aged generally sux as a rule, and sleep apnea left untreated makes is suck double. Pathological wakefulness can be a sleep disorder, it can be unresolved 'issues', it can be a cruddy mattress, pillow, sheets, sleeping clothes, too warm a room, too cold a room, drug effects, undiagnosed heart arrhythmias, GERD, pulmonary and tracheal defects or disorders, etc, etc, etc.

Honestly, having spent a lot of time on apnea fora, it's hard to tell how many people don't do well by CPAP therapy, or how they do without it if an authority sez they need it. Many complain of brain fog during the day, even when their morning reports show salutary numbers indicating good treatment on their machine's display. On the other hand, a great many start their treatment, it gets tweaked to improve it slightly, and they're off for the rest of their natural lives. I am one such person. I know of people who tried it, hard, for several months, and then gave up PAP therapy entirely. Or, they want to do it, but find in the morning that they ripped off their mask some time during the night. Well, it takes no brains to understand that thereafter they received no benefit, and may have continued their slide into really poor health.
In my own case, except for air blowing out my slack lips as I slept (meaning the pressures were poor), it has worked very well. I resorted to placing medical quality tape across my lips. It sounds draconian, but as an ex-soldier, you learn that you can learn, and do habitually, almost anything if the need is great.
Finally, some people can rise, void, think a bit, and fall back asleep. Almost all of us find that, the later it happens, the more unlikely it is that we'll return to sleep. Happens that way for me. This morning, after about five hours, I awakened and tried for an hour, but no joy.

I am a sleep junkie. I always have been. Sleep was my friend; solving emotional (occasional mild downers) or physical (colds, flu, etc.) problems. My greatest pleasure was sleeping till my body told me to get up (Usually on weekends). If I woke groggy, I thought I just needed more sleep. Retired from work, I moved to a new place and was never able to get into a regular routine of my 3 miles per day (running in younger days and walking later on) that I did every morning --rain, dark, sleet, or snow. I started to have very tired mornings and difficulty getting up out of bed and what I called "Fuzzy Brain" during the day. Thinking there was something wrong with me, I looked for a medical problem. I have had a lot of tests. I was surprised to find that I have mild sleep apnea. But more than that, I realized that though I still skied in winter and hiked in summer, I was not getting the regular exercise I needed. So now I go (not every, but most) mornings for a 2.8 mile walk on the desert. Though I am trying to get used to the CPAP for the apnea, I thing the biggest contribution to feeling better is the regular exercise. The CPAP has rehabbed my sinuses. So I am grateful for that. It has pretty much cured what I call "My old lady drip" that used to require a constant supply of tissues. I am 81. I am now signed up for another test to see if the CPAP is curing the low oxygen levels I have during sleep. That is of concern as low oxygen puts a strain on the heart.