I have set the game's randomTickSpeed to 0, summoned baby animals, then set the game speed to 1000, compared to the game's normal 3. After about 10 minutes, no animals have grown up.
MC-96184
The "Random tick speed rate" game rule does not affect mob aging
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randomTickSpeed affects only the ticking of Blocks, not the ticking of Mobs, see http://minecraft.gamepedia.com/Gamerule#gamerule and http://minecraft.gamepedia.com/Block_tick#Block_tick