A boy who is 21 years old and a girl who is 18 years old can jolly well marry as per their choice with opposite gender which is what Law of the land implements.
The sentiments and emotions read with dependence questions and capacity or maturity of both intending parties brings on board such emotive issues as you questioned us. Naturally any parents have such questions which needs tricky yet patience handling.
Otherwise so if you are 18 years old and has capacity to give your hand for marriage without force / pressure / fraud you can marry any adult person of your liking which is Law of the land and only thing for your own good self to think loudly is, if such person can keep you happy read with your happiness. All these come with maturity too. Feel the horizon and not only the view i.e. only the landscape and then act!.
Now the quality / quantity of happiness also differs from capacity / person to person / circumstances / environments so make a wise wholesome choice and take the informed leap without such parental pressures as such signed confessions / statements are not legally tenable documents for any present or future purposes by any person known / un-known.
Gist meaning hereby; any document signed with force / pressure / collusion / fraud / deceit / emotions – sentiments are never tenable in eyes of law irrespective of age / gender of signatory.