I understand your discomfort. I think it's related to the "mental age" of the person with a learning disability which means that we tend to think of people, e.g. with Downs Syndrome, as children. HOWEVER as I understand it, this is an outdated way of looking at things. Legally, someone with learning disabilities is able to consent at age 16 in the same way that anyone else would. (NB I know her modelling isn't about sex but the article is clear that she is looking to portray herself as "sexy" so I think it's relevant).
Whatever the law, I would hope that she has been supported in making an informed decision about how she wants to portray herself and it sounds like she has.
Sexuality and learning difficulties is a really uncomfortable subject, but I find it helps to remember that every adult has differing levels of maturity and needs to learn to approach their sexuality and sexual behaviour in an appropriate way.