I don't think there is a one-size-fits-all answer here.
Most people are best off getting pregnant between 18-35 and prime biological age is thought to be late teens to mid twenties.
I have two children, nearly 5 years old, and 5 months old, both conceived naturally - I'm 41. I had easy pregnancies, easy home births, two normal, very healthy children - b___stfed first exclusively for 7 months, the first finally stopped b___stmilk snacks at nearly 4! Second now being exclusively b___stfed. I mention this because my son has had one cold just after his 4th birthday - apart from that neither of them have ever been ill. These are very alert, strong children. I've had a healthy lifestyle throughout my life - so this could make a very big difference. Also my partner is 7 years younger - statistically a woman over 40 will find it easier/quicker to get pregnant when her partner is under 40. It took one month to conceive my first child and 4 months to conceive the second - the same rate as a woman in her early twenties.
Before 18, some people find pregnancy and childbirth difficult (long labour is common and problems getting b___stfeeding going) but many won't have problems of any kind.
After 35 most people may find it takes longer to get pregnant, particularly if you have an older partner, or if either/both of you has picked up problems due to sesxually transmitted disease (often have no symptoms) or lifestyle or exposure to xray/certain meds/chemicals (including household ones in many cases) etc. If your mother/her mother had an early menopause, you're best off making an early start as chances are very high the same will happen to you.
The problem for most women is knowing your own fertility as an individual. If your mother/her mother/her mother's mother etc had children over 40, chances are high you will have a long shelf life:) But tobacco/xrays/s_xually transmitted diseases (especially chlamydia) etc can make things different for you. An older partner will also reduce the chances of getting pregnant easily if you're over 35.
So the safest starting time is probably by the late twenties.
Under 16 isn't the best idea as your own body is usually still growing and things can be rough and difficult, but again, many young mothers escape these problems.
It really depends on the individual.
The main thing to remember is that all women, of all ages, even over 50, will usually have a healthy child, even though the statistical rate for birth defects goes up enormously. (The bigger problem for older women is usually be getting pregnant and staying pregnant past the first two months) For example, statistically 90% of babies born to 50-year-olds will not have Down's (10% will) - 40-year-old mothers - 99% of their babies will not have Down's (1% will). Most babies with birth defects are born to mothers under 35 because they have most of the children born, and because they're not offered routine checks for defects during the pregnancy. But at 28 your risk as an individual are much less - 999 out of a thousand mothers this age will not have a Down's baby.
There are different kinds of individual in other ways too.
Some very young women in their teens have motherhood as their biggest dream and are mentally well prepared for looking after children, but most will want to do other things first.
Some older women look forward to finally seeing the last of their brood leave home and won't want to start again with a new baby, whereas others crave another child to raise more than anything.
And some older women, like myself, are late to do everything, including finding a partner. For some, the only way to have a child in time might be as a single mother!
So, I think there's lots of different factors.
Usually women at a normal weight with a good, well-balanced diet with plenty of fresh fruit and vegetables conceive more easily and have healthier babies.
One thing I would advise is to think about your lifestyle. If you're late twenties and have any of these issues - overweight, smoking, drinking, on drugs, had unsafe s_x, need meds every day for a chronic condition, or ever had anorexia - your reproductive life could already have been shortened, so starting as soon as you can might be a big help - try to take the time to get healthy first to have a healthier baby. If you don't get pregnant in the first 6-12 months of trying, get checked out by the doctors as any problem can be treated much more easily at this stage.
Lastly, to show how much difference is out there, I saw a fertility clinic doctor on the TV saying that the greatest difference in a woman's egg quality occurred at age 41 in his clinic - some women still had absolutely perfect eggs, others were completely unfertilizable.
Best wishes to everyone trying to conceive - don't forget to get your PARTNER checked out if you're having problems - at least one third of all problems are caused by something going wrong with his fertility.