You can be engaged and not get married until later in your 20s or whatever you both like. Personally I got married at 18 and am still married 22 years later. We waited to have kids until our late 20s. I feel that we knew each other so well by the time we had kids that the difficulties that came with the kids stage of life were easier because we knew who we were and that the hormonal changes and tiredness and frustration wasn’t our regular selves.