A twice-born person, without having the Upanayana within the prescribed time, is called a Vratya in the scriptures. He is treated like an uninitiated person (a Shudra) although he might be a Brahmin by birth.
There are some remedies available for him (like the Vratyastoma Yajna) performing which he can regain his status of a twice-born.
71 For a Brahmin the time for initiation does not lapse until the
sixteenth year, 72 for a Ksatriya until the twenty-second year, 73 and
for a Vais´ya until the twenty-fourth year. 74 After that time they
become excluded from the Sa¯vitrı¯;
75 and no one should initiate them, teach them, officiate at their
sacrifices, or enter into marriage alliances with them.
76 A man who is excluded from the Sa¯vitrı¯ should perform the
Uddalaka penance: 77 for two months he should live on barley gruel,
for one month on milk, for a fortnight on curd, for eight days on
ghee, for six days on food received unasked, and for three days on
water; and he should fast for a day and night.
78 Alternatively, he may participate in the ritual bath that concludes
a horse sacrifice; 79 or he may perform the Vratyastoma sacrifice.
Dharmasutras of Vashishta 11.71-79
What happens if they marry?
Nothing will happen. They can marry but then the sons fathered by them will also be Vratyas who are not qualified for initiation (Upanayana).
16 Sons fathered by an uninitiated man are Vratyas, excluded from initiation––so state the wise with equal reference to the three
Dharmasutras of Baudhayana 1.16.16
I am not sure though if the remedies mentioned in the first quote can be performed after marriage or not possibly not.