Gelatinous Cubes mature at the same rate and live for about the same amount of time as humans.

Gelatinous Cubes usually tend towards law or neutrality.

Gelatinous Cubes have 60 ft Blindsight (they are blind beyond this radius). 

You take up your entire space. Other creatures can enter your space, but a creature that does so is subjected to your engulf and has disadvantage on the saving throw. Creatures inside you can be seen but have total cover. A creature within 5 feet of you can take an action to pull a creature or an object out of you. Doing so requires them to make a DC 12 Strength check, and the creature making the attempt takes 2d6 acid damage. You can hold only one Large creature or hold up to four Medium or smaller creatures inside of yourself at a time.

Even when you are in plain sight, it takes a successful DC 12 Wisdom (Perception) check to spot you if you have neither moved nor attacked. A creature that tries to enter your space while unaware of you is surprised by you.

Whenever you enter another creature’s space, the creature must make a DC 12 Dexterity saving throw. On a successful save, the creature can choose to be pushed five feet back or to the side of the cube. A creature that chooses not to be pushed suffers the consequences of a failed saving throw. On a failed save, you enter the creature’s space, and the creature takes 2d6 acid damage and is engulfed. The engulfed creature can’t breathe, is restrained, and takes 4d6 acid damage at the start of each of your turns. When you move, the engulfed creature moves with you. An engulfed creature can try to escape by taking an action to make a DC 12 Strength check. On a success, the creature escapes and enters a space of its choice within 5 feet of you.

Melee Weapon Attack: reach 5 ft., one creature. Hit: 2d6 acid damage. 

You have resistance to poison damage, due to your gelatinous form.

Before being able to use a language, you must engulf a creature with a mouth, vocal cords, and limbs. After doing so, you can speak, read, and write Common and Cubic.

Your Constitution score increases by 2, and your Strength score increases by 1.