From what you've said it sounds very likely that her mother is the onewho will try to persuade you to wait until your beloved is older.
Sinceit sounds like mom, not dad, will pose an obstacle, why not have aprivate talk with her mom? Ask your beloved if she is comfortable withthis. Also ask her if she'd like to be present for the discussion. Whenyou talk with your soon to be mother-in-law, tell her exactly whatyou've told me; that you adore her daughter and that you both want tostart a life together. Tell her that you that she wants her daughtersto go slow.
Tell her that you understand she believes that having hergirls live independently until age 30 or so will protect them. Ask herhow she thinks waiting will protect her daughters. Will they know howto take care of themselves in case their husbands leave them? Tell herthat you have no intention of leaving her daughter. In any case, youwon't interfere with your beloved's ability to develop herself. Tellher that, in fact, a healthy relationship shouldn't eclipse the other's independence, and you want to encourage your beloved's independence.
Then tell her, you both want to develop as individuals but don't wishto put your lives or love on hold. Tell her that it's important for hermother to support her daughter's independence by allowing her to makeher own independent choices. This is practicing what she preaches!
Finally, tell her that you care very much for her and want her supportand her blessing. I want to hear how you make out.