- Does your child meet the weight, height and age requirements?
- Can your child remain properly positioned for the entire length of every ride, never leaning out of position?
- Will your child leave the seatbelt properly positioned flat across the collar bone and hips/thighs?
- If you're not sure about your child's maturity abilities and/or your child still fits a harness, then keep him or her in the harness.
- If your child has outgrown his or her current forward-facing harness, but is not mature enough to use a booster properly, there are various alternatives listed HERE
- Advantages to Extended Harnessing include:
- Limits head excursion (forward movement) during a crash, thus reducing the risk of brain + spine injury
- Spreads crash forces out over a broader area of the body at all of its strongest points
- Keeps kids properly positioned much more reliably
- Helps prevent backseat bickering between siblings (thus reducing driver distraction)

Although some manufacturers may state that any child over a mere 12 months and 30 pounds may use a booster, this does NOT meet safest practice according to crash data research! Toddlers should never ride in a booster. Cognitive ability associated with the necessary attention span to use a booster correctly does not even begin to develop until at least 4 years old. Prematurely "graduating" a child to a booster before the minimum recommendation of 4 years and (not or) 40 pounds puts the child at increased risk of injury from "submarining" and also puts the driver at risk of distraction, increasing the risk of crashing.

Use of 4 or 5 point harnesses have been a standard safety measure for grown adults in the racing industry, for pilots, astronauts, etc.
If a child still fits a harness seat according to the limits listed in the manual, then that child should not yet switch to a booster. A booster is only safer than a harness if the harness cannot be used correctly and then only if the booster will be used correctly. (If a child no longer fits a combination seat's harness limits, then he/she must use it as a booster at least until you have a more appropriate harness
*).
Remember that each "graduation" in seat type is actually a demotion in safety. If a child in a booster still finds that the shoulder belt crosses the neck/face rather than lying flat on the collar bone, then this child needs a high weight harness seat. If a child cannot or will not use a booster correctly for the entire length of every ride, then that child needs a high weight harness seat.

Don't you want to keep your young child harnessed at least until their skeletal structure is more developed? Preschoolers' skeletal structure may still be quite flexible, increasing risk of internal injuries especially since 3-4 year olds are more likely than 5+ year olds are to submarine down/out of boosters....
Remember: the pelvic bones do not finish developing until puberty! It's just not worth the risk to diminish your young child's chance of survivial by switching to a booster prematurely when there are options out there that will reduce the risk of injury by staying harnessed until kiddo meets both physical + maturity requirements for a booster.
Tip: Switchable/Automatic Locking Retractor (ALR) seatbelts can help remind wiggly kids to remain properly positioned in a booster, but some kids are still able to squirm out of position. If your vehicle does not have Switchable/ALR seatbelts, it would be safest to keep a young child in an extended weight harness alternative or the Recaro Young Start booster that has optional manual built-in shoulder belt lock-offs.
* using a harness beyond the limits is MISUSE, which increases the child's risk of injury.