I’m not a numeray, but an associate of Opus Dei (a celibate member who normally lives with family member or by oneself because of permanent personal, family or professional circumstances that makes one unable to take on the tasks and availability of numeraries who normally live in a Center). I think I can answer your question though since like the numeraries, the associates also live the virtue of poverty in a similar way (i.e. turning in the salary and then asking for whatever one sees that she needs from a “common pot”), I join Opus Dei when I was 20 (then about to finish my undergraduate degree) and I went to medical school afterwards (still living at home and dependent on my parents for my allowance, and an aunt generous enough to fund my medical school studies). I can imagine that the college-aged numeraries or those who did graduate studies lived poverty just like I did that time.
Instead of turning in my salary (which I did not have at that time), I turned in my allowance and any other money that was given to me by family as gifts or money to buy spend on projects, etc. aside from my allowance. Then, I asked for what for what would cover my usual needs or consult when buying something extra like a new pair of jeans, for example.
I was also encouraged to try to find another source of income, which I did for only a short time, ghostwriting for an advice column on-line. I found it difficult though to juggle with medical school studies so I had to give it up eventually.
If you’re wondering, I’m now about to turn 30, and already earning a modest salary while doing my residency training.