Yes, that sentence is certainly understandable.
It's a little awkward, but I think it's mostly an awkward phrasing because it's a bit of an awkward thought.
This isn't related to tense, but if I were to change something about that sentence, it would actually be the assumption that getting married equals having a daughter. I would instead say:
If I had had a daughter ten years ago, then she would be eighteen eight years from now.
or, since being single is part of the regret
If I had gotten married and had a daughter ten years ago, then she would be eighteen eight years from now.
However, none of these sentences actually capture any of the regret you're trying to express. They just state a mathematical fact. Maybe something more like:
If only I had gotten married ten years ago, then I could have had a daughter who would be eighteen eight years from now.
'If only' expresses regret, saying that you wish that this other situation had happened. 'I could have had' indicates a missed opportunity for something that you would have liked to have, but now can't.