Shares of Tencent Holdings Ltd. and rivals fell Tuesday after a state-owned Chinese newspaper criticized online gaming as “opium for the mind," fueling investor concerns that the companies’ popular games could be swept up into a broader regulatory crackdown. Within hours the article was no longer accessible on the paper’s website, before later reappearing with some of its harsher language removed. Meanwhile, Tencent said it would introduce stricter curbs on younger users. Tencent’s shares, which had dropped more than 10% earlier in the session, pared some losses after the article disappeared to close 6.1% lower in Hong Kong at 446...