_flagUpdate () {
		const that = this;

		// Set flags to update (causes less garbage than
		// ```ParticleSystem.sortParticles = true``` in THREE.r58 at least)
		that.attributes.age.needsUpdate = true;
		that.attributes.alive.needsUpdate = true;
		that.geometry.verticesNeedUpdate = true;

		return that;
	}
Tells the age and alive attributes (and the geometry vertices) 
that they need updating by THREE.js's internal tick functions.

@private

@return {this}