Of course the most significant is April 16, 1862, the date for the so-called Conscript Act. The law went into effect east of the Mississippi River on May 16, 1862. Newspaper ads urged men wishing to avoid the stigma of "conscript" to enroll before that date. Young men reaching their eighteenth birthday and those not covered by the law (AKA exempts) would still be still considered volunteers. However, anyone else enrolled after May 16, 1862, would be a conscript.
That expression brings to mind a Jeff Foxworthy routine, but I'll let it pass.
http://www.fortogden.com/foredneck.html
You may be thinking about October 1862 because the upper age limit went from thirty-five to forty that month. Eventually it became forty-five, as established by the law of Feb. 17, 1864. Also, during December 1863, Congress repealed the law allowing citizens to avoid military service by hiring a substitute. In an interesting twist, men employed to serve in their place were still obligated to their term of enlistment.
Congress passed a furlough and bounty act in late 1861. This provided a $50 bounty for volunteers enlisting for the war, and in addition, a furlough for those reenlisting for the war. Those who had already served a twelve month term could count a years service against the requirement of three years or the war. For that reason you'll see the earlier enlistment date used when a veteran joined a new organization like the 7th Mississippi Battn. You should also see the soldier's term of service altered to read, "two years or the war."
The law wasn't clear on several points, so officers didn't know what to do with twelve-month men asking to reenlist, claim the bounty and get the furlough. Did a soldier have to honor his original term of service first, and then reenlist, or could he reenlist before his term ended? At Pensacola General Bragg allowed the latter, soldiers from Mississippi commands and other states joining a new battery under Capt. Felix Robertson and the 1st Confederate Battalion. Because Congress failed to consider the consequences when crafting this legislation, the law decimated veteran commands like the 9th and 10th Mississippi Regts.
Seems like some things haven't changed much.