Well, it's a bit confusing, 'cos there seem to be conflicting reports.

I always thought that in OoTP, Sirius is about 37, and Lucius is 41. But apparently the HP Lexicon says there's an age difference of six years, with Lucius as the elder of course.

The only things that seem to be certain are that they're not the same age and Lucius is older.

Hope that helps and good luck with the fic, the world needs more Lucius/Sirius. :)