nah you can start drawing any time you want! and it's a great thing to do as well, incredibly time consuming but once you start you're hooked:) or maybe that's just me cos that's all i ever do...
anyway personally books annoy the hell out of me in terms of learning to draw cos it's much more helpful having an actual person telling you exactly what's going wrong with your drawing. so i'd suggest finding a local art centre and take up drawing classes. they usually have classes for specific aspects such as portraiture. it'll cost you a bit, depending on where you go, but they're worth it, cos the teacher'll give you some good information on drawing and stuff, and personally i find it much more helpful hearing it from a person rather than a book cos it's more personal.
i started seriously looking at doing realistic drawing back when i was about 14 and was playing with tone and stuff. what i found that was good just to analyse the shapes of the various parts of the face was just tracing the outlines of the eyes and mouth etc just so i could see the general shape without all the crap in between- by doing that i discovered that eyes are not as big as i thought they were! it may seem like cheating but i found it a good way to start, and then i'd add in the tone and little details to that.
i could give quite a long rant about this, so if you want any help from me (my qualifications are just finished first year b.fine arts at uni!) you can specify aspects and i can give some tips and i can draw some examples if you want. i'm not the absolute greatest with pencils, i prefer paints, but i can give a few pointers:)
Topic: drawing.