There's a difference between a toddler seat and a convertible child seat. DD was 18kgs just before 4, but she's seriously tall.
You don't want a convertible from birth. You want one from 8kgs.
A convertible child restraint goes from birth to 18kg and can be used either rear or forward facing, a toddler seat goes from 8kg to 26kg and has a built in 5 point harness that can be removed once your child is over 4 years. A booster is from 14kg and doesn't have a built in harness. I used a convertible seat with both DSs from birth.