Anything with testicles is getting sent away fat if at all possible by August. Any lambs with testicles still here by September get speaned and kept away from the ewes. Hereabouts, we can give ourselves another month to separate them from the ewe lambs if we want. (We lamb in March for the most part, and I don't think we've ever had a hogg lamb before April.)
Factors will include climate (nothing got randy on our farm till mid-Oct last year, it was just too warm), breed (we find the Charollais and his offspring happier to get busy in August, whereas the mules and those with Blackie blood can't think about it before October, really), age (ours are 5 months by August, if you lamb in January they'll be ready to work sooner!), and fitness.