Of course it is not possible to fit wizarding Britain into the muggle trend of population change because wizards have magic. There is no reason to believe that just because modern Britain has a low birth rate and relatively small proportion of children, the same can be said of wizarding Britain.
So at what developmental demographic stage is wizarding Britain in?
JK Rowling originally estimated the number of students in Hogwarts to be around 1000 . Although this seems too high compared to the evidence we have in the books based on Harry’s year in Gryffindor, there is very little point in second guessing these figures because of all the things we simply don’t know.
The low number of students in Harry’s year may reflect a corresponding crash in birth rate at the height of Voldemort’s terror campaign and/or a magical epidemic. Gryffindor may be generally a smaller house than the other three. There is no reason to believe that the sorting hat does proportion students out evenly.
Rowling clearly intended Hogwarts, in the time frame that we see it, to be a big school even by muggle standards and children were supposed to represent a significant proportion of the wizarding world.
Many people will bend over backwards to disprove that Hogwarts is so large and the wizarding population so small because they wish to impose the demographics of modern muggle Britain onto Harry Potter’s world. This is entirely fruitless, as the wizarding Britain is a completely separate society developing along a completely different trajectory.
It may looked like a developed society by muggle standards but one cannot impose the definitions of muggle development: low birth rate/death rate, and long life expectancy, on a magical society.
These seemingly “skewed” figures may be the key to understanding more about wizarding society because there is no reason to believe it is structured in anyway like muggle Britain.
If the wizarding world only has about 3000 people according to Rowling , 1000 of which are children between 11-18, these students at Hogwarts make up 33% of wizarding population. As of the 2011 census children aged between 10-19 make up 12.1% of the total population of the UK. The demographics of wizarding society definitely do not resemble that of modern muggle Britain. In fact it displays the typical demographics of a pre-industrial society where birth rates and death rates are both high compared to the UK. In these societies children represent a disproportionately large proportion of the percentage, and adults and the elderly a much smaller percentage.
However whilst the demographics of wizarding Britain may reflect modern day very developing countries today a better parallel to draw is pre-industrial Muggle Britain in the 16-18th century, precisely around the time when wizarding Seclusion was being introduced.
In the mundane world as societies develop, their demographics change and they transition through 5 distinct phases where there is first, a large reduction in death rate due mainly to medical advancements and then a corresponding drop in birth rates due to contraception and changes in societal attitudes. The life expectancy rises and the elderly end up comprising a disproportionately large proportion of the populations.
We know that wizard societies cannot develop and transition in the same way as muggle ones. The wizarding world is never going to have the equivalent of our public health revolutions that decrease death rates dramatically because they already have clean water and sanitary living conditions. They have had these things long before their muggle counterparts and the consequences of this must already be reflected in the current demographics of wizarding Britain. They must still have a high and fluctuating death rate despite good living conditions. Clearly the lethal problems facing wizarding society in the UK are different to those faced by modern developing nations. Good food, clean water, and adequate housing are not going to solve their problems.
So why does the wizarding world resemble Britain in the 17th century?
 Students at Hogwarts - "About the Books: transcript of J.K. Rowling's live interview on Scholastic.com," Scholastic.com, 16 October 2000
Rowling "There are about a thousand students at Hogwarts"
 Population of the Wizarding Britain -"The Leaky Cauldron and MuggleNet interview Joanne Kathleen Rowling: Part Two," The Leaky Cauldron, 16 July 2005"
Rowling: "Let's say three thousand [in Britain]" (discounting the various magical creatures/beings)..."actually, thinking about it, and then think of all the magical creatures, some of which appear human. So then you've got things like hags, trolls, ogres and so on, so that's really bumping up your numbers"
Continued in Part 2 - Magical Maladies