Confusing, to me, are the characters' ages and backstory

Each time I watch this, I can't get around the unconventional ages of the Giddens. The subject of aging is clearly broached, and often, but Regina seems nothing more than about 40. To some of a certain age, that may seem old. At my age, it's not, really. But I can overlook that as a form of vanity she struggled with. On the other hand, Horace seems distinctly older than Regina. Is he supposed to be? Or is the actor just appearing older?
Finally, Alexandra's age/persona is all over the place. She's played naively and timidly, including her dress, like she's no more than 12 or 13 but she must be older than that. She wouldn't be "courting" at that young age in 1900 nor realistically allowed to travel by gritty public transport unless she was at least 16. It might be an anachronism created by how she was directed to play the daughter or a reflection of the period I'm not familiar with. Combined with the antagonism of the leads, I'm surprised they even had a child and so late. Does anyone know their intended age? And, if they know why the two parents were even married, given what Horace feels about her and her family? He must have known what he was getting into, somewhat.

I love this film but the Horace's vague backstory has always bugged me. Birdie's past and how she became trampled by the family is much clearer.