See, you could be right, but there are various indications that Zed was quite a bit older than 20 when he met Kayn. The first important evidence of this is the 4 year investigation to stop Jhin. Zed was described as Kusho's "star apprentice", suggesting that he has been training under Kusho for a number of years. I wouldn't expect him to be too old, but he was likely in his late teens at the youngest - ~18, let's say. I mean, he would have to be old enough and mature enough to take on a long-term hunt for a vicous psychopath. After the investigation, he would have been in his early 20s - 22 under the earlier assumption.
After the Jhin incident, some amount of time passed in which Zed peeked in the magic shadow box and got kicked out of the ninja academy - assume maybe a year, so he's 23 at this point. Upon being exiled, he wanders for years - so at least 2 - before he even begins training others in the shadow arts.
We know from Kayn's lore that the Order of the Shadow was already well established when Zed took Kayn in (it was known that acolytes would train for several years to master a single weapon, indicating that there had been enough time for that to be known as standard). From Zed's lore, it appears that Zed's group wasn't officially known as the Order of the Shadows until he killed Kusho and took over the ninja academy. Therefore, we know that the Kayn incident must have been another few years after killing Kusho.
So, assuming a year or two in exile, then a couple years training his first followers, then probably about 5 years before meeting Kayn, we can assume that Zed was at least 30 before meeting the boy, putting him in his late 30s or early 40s now.
Nidalee has white marks all over her body 0:
