Depends on how fast/easy you level. Personally when levelling alts I stick to my 65 SAs until I can use a 75 SA, but if you think you need more DPS it can be worth it to get a level 68-72 third age weapon inbetween (for class items the level isn't that important). If you can't (or don't want) to get a 75 SA weapon I'd probably get a new third age weapon whenever my current weapon is around (item-)level 50 and level it until my current weapon is maxed before equipping it.
But as said, it's very dependant on your personal playstyle.
Used to play: 85 Champ / Captain / Runekeeper / Guardian, Guild Master of everything but cooking.
Playing now: Hellcat / King Tiger / GW Panther / IS / KV-5 / M4 Sherman and more