1. The Low E bass clarinet manufacturer tells you that the alto clarinet has bE and F keys. The modified basset is in the key of F, a major second higher than the alto clarinet in bE, and is a slightly smaller bass clarinet with the bell pointed upwards.

2. The Low E bass clarinet manufacturer tells you that the bass clarinet is an octave lower than the treble B-flat clarinet, similar in shape to a saxophone, with a curved whistle pipe and a trumpet facing up, and a treble staff for notation, which is the standard for military bands musical instrument.
3. The Low E bass clarinet manufacturer tells you that the double bass clarinet is in the key of bB, which is one octave lower than the bass clarinet, which is relatively rare.
Features of clarinet:
The Low E bass clarinet manufacturer tells you that any age after your child has changed his front teeth is suitable for learning the clarinet. 
The above is the Low E bass clarinet manufacturer's introduction and analysis of the bass and alto clarinet, hope it helps you.