Why minimum weight exists
Weight sensors are part of the safety system on many automatic litter boxes. The manufacturer validates automatic behavior around a defined operating range. A cat below that range may not trigger the system in the same way as an adult cat.
That means “the kitten fits through the opening” is not enough. Physical fit and sensor eligibility are separate checks.
Do not copy thresholds between models
Different products use different sensor arrangements and software logic. Use the exact current manual for the model you own. If a company changes hardware or firmware, old advice from forums can become stale.
Manual cycling can be the bridge
Some manufacturers provide a supervised manual mode or kitten mode below the automatic threshold. Follow those directions literally: verify the cat is away from the machine before starting a cycle, and do not improvise around the safety system.
When to transition to automatic mode
Use the manufacturer’s published minimum and any setup instructions, not age alone. Body weight can vary widely between kittens of the same age. If you are uncertain whether the pet or product is suitable, contact the manufacturer before enabling unattended cycles.