Gosh,I think the OP has come in for some somewhat unfair comments.
I think a woman who is sure she doesn't want kids is definitely to some extent or other lucky.
And not just because study after study shows that having children doesn't increase ones happiness overall.
The main disadvantage as has been stated is probably the increased likelihood of loneliness in old age.
But you know the greatest loneliness of all is arguably that particularly acute loneliness that is only felt by women in unhappy marriages.*
And having children doesn't necessarily insure you against suffering it.
I always knew I didn't want children and for me it was simply a matter of not wanting to bring life into a world I have always seen as simply too awful.
When you think about all the suffering that each extra human being causes to countless animals - and often to each other!- throughout their lifetime, one way or another, and the amount of rubbish each extra person produces in their lifetime and the amount of resources each extra person consumes in their lifetime, then not bringing another child into this world is easily the greatest 'contribution' an ordinary person can make to this planet I very much believe.
Because of course it's usually not just a case of one extra person but 1000s and 1000s of extra people if they and their children reproduce.
If such a person is also able to be satisfied with living in a flat instead of a house then they are also helping to alleviate the housing crisis to boot.
And if they are happy to go relatively early to boot.....
Thus I wish the OP the very best of luck........
* I have possibly read too many novels by female authors especially of a certain age.