The bare minimum for a booster seat is 4yrs AND 40lbs. Your child needs to be big enough for the shoulder belt to lay across the top of the shoulder and middle of the chest, for the lap belt to lay low on the hips (and NOT across the abdomen), for the knees to bend at the edge of the booster seat. The child has to be mature enough to sit upright the entire trip, not fall asleep and slump over, not put the shoulder belt behind their back, and not bend over, lean, or reach for things. Its best to use a dedicated high back booster seat, instead of using the booster portion of a 3-in-1 seat like the Alpha Omega. The Alpha Omega and similar all failed miserably in every crash test because they are too wide and the belt path places the lap belt across the abdomen.
Children under 4 and 40lbs are 75% more likely to suffer head and abdominal injuries in even a minor crash, and to submarine under the lap portion of the belt.
A harnessed car seat is outgrown forward facing when one of three things happens; the child goes over the forward facing weight limit, the ears become level with the seat back, or the shoulders go over the top harness slot.
A high back booster seat is optimal for a child under the age of 6. It provides side impact protection, belt guides for the shoulder belt, and its more secure for younger kids.
If your child is under 4yrs old and is outgrowing his/her harnessed car seat, there are so many options out on the market right now that will keep a child in the harnessed until a more appropriate age.
Evenflo Generations65 ($99) will harness to 65lbs, and then becomes a high back booster. this is a great seat, and it make a pretty good booster seat.
Graco Nautilus ($150) will harness to 65lbs and then booster to 100lbs. It also becomes a no back booster if necessary. It looks like a high back booster seat, and this seat is a GREAT seat for the money. Most kids can use the harness until they are 6/7yrs old. I really recommend it.
There ar ea couple more, but these are the two that make great harnessed seats for older kids and then become good booster seats for big kids.