I've had low self-esteem my entire life, but my older sister does not. Part of that, I think, is due to our having completely different personalities - my sister is extroverted and outgoing and is going to do what she wants to do while going through life with a happy-go-lucky attitude. I'm an introvert, shy (though that has gotten much better with age), and have always felt like I needed to put everyone else's happiness before my own.
Birth order may have had a small part to play in it. My sister is the oldest and was the first grandchild on my mom's family's side, and, as such, was the center of attention and had a normal amount of fuss made over her. In essence, she had the groundwork laid for good self-esteem - she felt she was important in her little universe. By the time I came along, not so much. I pretty much felt invisible.
Another aspect in my family is that my mom treated me completely differently than my brother and sister. My sister was golden because she was first-born, and my brother was golden simply by being a male child (big deal in Italian families). I was the youngest, was very sensitive, was alone at home with my mom once my brother & sister were in school, and very quickly fell prey to my mom's own issues with low self-esteem and narcissism. With my personality and those conditions, I never had a chance to develop a sense of self-worth or self-esteem as a small child because there was no one to teach me or mirror that to me. My most important mirror (my mom) told and demonstrated to me daily that I had no worth and nothing I did was right or good enough.
The most improvement I've had in my self-esteem has been from being a part of this board and interacting with all of you -- you've all been my role models, and I thank you from the bottom of my heart.
I do think that self esteem has always been a problem; it just wasn't identified as such.