There is a vast range in the rates at which children grow, as well as in their final adult sizes. So there is a great range in the possible size of your 14-year-old boy or girl, and in how old they look.
Some 14-year-old more or less immortal kids might look much younger than 14 to most people who see them. Most would look sort of like 14-year-olds to most people who saw them. Others might look like full grown adults to the people who see them, which I suppose can be ruled out for the purpose of this question since it makes having adult careers easy.
Suppose that the immortal 14-year-old was a member of an aristocratic or middle class family. If their father died while they were still a child, they might come under the official guardianship of their mother or a male relative until they came of age. In England the legal age of majority was usually 21, while in France it may have been 14. presumably his birth date would have been written down (in cas ehe ever wanted tohave his horoscope read, for example) and so there would be an official date for the end of his guardianship.
And once he reached his age of majority, and people were noticing he was small for his age, he might travel to a foreign country with a companion who he might pass off as his guardian in the foreign country, where he might claim to be a 14-year-old child. Having someone pose as his legal guardian would give that person authority over him, but if the child was much stronger than most kids his age and even than adults he could intimidate the "guardian" into not abusing his fake position.
Possibly the "guardian" might have a thing for 14-year-old kids and might consider it ideal to pose as the "guardian" of someone who was eternally 14 years old physically.
And the kid and his "guardian" might travel from country to country or city to city every few years so that the kid's lack of aging would not be noticed.
It is possible that an eternally 14 year old girl might also be able to do the same thing.
Of course as a girl got into her later teens and her twenties her family might want to arrange a marriage for her. And how acceptable a bride she might be for prospective grooms would depend on the size of dowry offered, her degree of beauty, her personality, and if she seemed capable of child birth. The more years older than fourteen she got while still physically fourteen, the less likely she would seem to grow in the future.
So if someone was considering marrying a girl who had looked 14 years old for years and didn't seem likely to grow any more, he might have her examined by doctors and midwives for their professional opinions about whether she seemed to be built for safe and successful childbirth.
Every person alive today is descended from countless millions and billions of mothers, many of whom gave birth to healthy children when aged 14 and even younger. But I think that only a minority of fourteen year old girls are properly developed for childbirth.
So an eternally fourteen-year-old girl's marriage prospects would depend on whether she resembled a typical fourteen-year-old girl probably not developed enough for safe childbirth or a rarer fourteen-year-old girl who seemed capable of safe childbirth.
I note that in the Age of Discovery most large European ships' crews would have a small proportion of boys aboard.
An eternally 14-years-old boy could join a ship sailing to some exotic foreign land, and there he might join the crew of another ship, and another ship, and another ship, over and over again, claiming to be 14 every time. If the boy reached a polytheistic land like India or Indochina, or China, Korea, or Japan, he might try to found a cult with him as some sort of immortal child and minor god.
And in Europe someone might found a monastery if they have enough money, and perhaps make unusual regulations such as the monastery always has to provide room for children of the founder's family who will from time to time be disciplined by being sent there for solitary confinement, seeing no one and thinking about what they did wrong. Each time a hopefully repentant child finishes their term of confinement and leaves for the family home, another trouble making child will soon arrive, both wearing hoods and holding their heads low in shame so no one can see their faces.
History is full of examples, with varying degress of success, of women passing themselves off as men, men passing themselves off as women, boys impersonating girls and girls impersonating boys. Possibly your ageless boy or girl, might alternate male and female fake identities.
It seems to me that a typical looking 14-year-old boy could pass himself off as a young woman who is unusually small in size and rather average looking if he wears women's clothing. So possibly he might get a job as a tavern wench and possibly slowly work his way up to owner of the tavern, or some other job for women. He might get a cottage and till a garden and spin thread at home for sale, etc.
If he looks like 14 forever, he might have to start wearing a mask to hide his face after some years, explaining that he had a disfiguring accident or illness and can't bear the though of anyone seeing his face.
And no doubt there are many other methods for an eternally 14-year-old child to conceal the fact that they never age.