Do we age or mature?

My guess is we mature and then we age.
As children we mature into adults and will hopefully sustain a suitable level of maturity, virility and health for some years before the ageing process takes effect.

How long can we hope to maintain a decent level at our peak of health?

My guess, this surely varies from person to person, depending on genetics, lifestyle choices, food, lovers, friends, how we take our misfortunes and of course luck. Our peak could start at any stage of our lives, possibly as young as twelve years, our late teens or even much later and curtail again at varying times.

What causes ageing?

My guess is stress, tension and illness, worrying about yourself, your partner, your parents and your children. Quite simply the more you care, the more stressed, tense and worried you'll be, causing you to age quicker. Your parents will one day need you to care for them, just as you are stressing to care for your own children. In a world in which money worries are predominant and continuous and employment is sometimes unstable.

I believe the loss of the most important person in our lives (especially in our later years) will usually dramatically speed up the ageing process as we mourn our loss.

Remember also, crying causes the face to crumple, the shoulders to drop, the head to slump, the back to bend and the knees to buckle. Not a position you'd wish to remain in, but too much of this may cause the body to hunch and alter your stance for the worse. Nobody cries with their eyes wide open, their head held high, their shoulders square and their backs straight unless they're strange or having a laugh.

Too often we're told we're too old to do certain things anymore. RUBBISH
Those same people will simply let themselves go before time and stop spoiling, preening and pushing themselves forward. With no real reason to get up in a morning, they're more than happy to drag you down with them.

The way we walk, stand and sit will too often alter as we age, encouraging aches and pains in various parts of our bodies, causing many visits to our doctors then hospitals for treatment and pills too often with no or little effect, with nothing to look forward to but discomfort and sleepless nights.

Some people you'll notice seem to look better as they get older. Well into their 60's they are buoyant, bubbly and ready for the next challenge. Always vibrant and pristine day or night, despite having to suffer the same misfortunes everyone else endures.
Ageing especially noticeable once you hit 50 years of age when so many loved ones will fall sick and pass away before your eyes. Remember your parents will be 70 plus at this time, as will your uncles, aunties and cousins. Grandparents if still alive will be now surely bedridden needing your comfort and assistance, not to mentions your partners family and your friends who have all hit the ageing period. All draining your resolve and corroding your time and effort.
You'll grieve for your own, you'll grieve for you partners family, you'll grieve for all the people you've known and loved throughout your accumulative life. That's alot of grieving that has the potential to stress and age you beyond belief, as you're wary of more to come yet and thus age yourself to the confines of your home. Making you ill and placing you into the hands of doctors, hospitals, nurses and prescription drugs. Surrounded by other sick and dying people unable to leave their beds unassisted. Pills for this, pills for that, crossing and combining to confused, complicate and contaminate your system. Operations weakening your resolution, your resistance, your confidence and reminding you your time will come soon if you don't stay positive, spruce yourself, smile more and live for today no matter what life keeps throwing at you.