//why have an age limit on IK but not Lotto or DK in the first place?

Because Instant Kiwi is a much more insidious form of gambling than at least Lotto (I don't know anything about Daily Keno, so can't really comment).

In behaviourist principles, the delay between stimuli (buying ticket/scratching) and the reinforcer (prize, if any) is very short (if you scratch it in the shop, perhaps a minute or two). In contrast, buying a Lotto ticket involves waiting until Saturday night, and then at least a 12 hour delay from finding out that you've won something to actually getting the prize. This shorter delay means a stronger relationship between buying and scratching the ticket and getting the money. Thus, Instant Kiwi buying can become a stronger habit faster. (A different example of the same phenomenon: If a kid/animal does something wrong, the quicker you reprimand/punish/whatever, the more likely it is that the "wrong" thing is perceived as bad. Only dishing out discipline at 8 o'clock on Saturday is not likely to be as effective).

The next phase of insidiousness is that having won on Instant Kiwi, there is a stronger likelihood of reinvesting all of your winnings. Ie, if you win 20 bucks on Instant Kiwi, you're more likely to buy 20 bucks more IK. However, if you win Lotto, because it's more of a weekly habit, you'll probably just buy your usual tickets (or maybe one extra) and pocket some profit.

In these ways, Instant Kiwi is more like a pokie machine. The short delay between playing and winning makes it more addictive, and that rather than having got lucky once, that fate is on your side. You then reinvest all of the profit, and continue to gamble until the money is gone.