Speaking of time "по" means that the following number or other marker is included is the time interval. "Я работаю с 9 до 17" means that you stop working at 17. But "Я работаю с понедельника по пятницу" means that you actually work at Friday. The second example is difficult to comment without knowing the context. But, it seems that it is used to express diminishing of a certain parameter (say, average number of kids in a familiy?). In this context "по" would sound wierd.

Louise по... is used with time units like days. It means, the day is included. до... works well with points, borders... До пяти часов. - With time uints lesser than a day, cardinal numbers mean a pont in time: 17:00. По is not needed here. C первой до третьей минуты. С первой по пятую минуту. - Ordinal number refers to the munute as a whole. Problem is that, по... is commonly met in various forms and documents (to avoid ambiguity). When you use it in non-formal speech/writing, it sounds a bit like you are used to filling forms/resumees. At least in Russian spoken around me. до... is much more common! It doesn't need you to take an effort to say it. По..., conversly, may need you to consciously be precise (unless you are used to documents). And до is often ambiguous. When you say "до пятнадцатого" people OFTEN ask: "включительно?" And answers vary:/ When you have a honeymoon in Caribbean islands from January 2nd till February 4th, and when you return back in the middle of the day February 4th you may not use по. Because: - not the whole of the 02.04 is included! По would be misleading here! For this reason we can NOT restrict до to 'till the beginning of the day'. - you are not taking effort to be precise (and aren't used to work with documents). When you are attending university "с первого января до второго февраля" you may use до just because you are not making an effort to be precise. You might mean: до второго включительно (because you have an exam второго февраля. Or you have seen those dates, 1 and 2. You repeat what you have in mind). по первое включительно ("до 2-го" because you are thinking about how you are free since the February 2, you are building plans). But you also may use по here. Because now that day IS included and because in the context of university you naturally switch to the official langaueg of schedules! Other useful way to disambiguate things is to say по/до второго включительно.

Hello Louise, When we talk about a period of time we may use "с ... по ..." or "с ... до ..." but the first option sounds better. For example: - Он работал в компании с 2011 по 2014. / He worked for the company from 2011 to 2014. When we talk about a child age we should use "с ... до ... лет". - Программа рассчитана на детей с 5 до 17 лет. / The programme is geared towards children aged 5 to 17 years.