Is it regarded as immoral to marry a young girl in Islam given that prophet Muhammad married Aisha when she was 6-7 and had her consummated when she was around 10? Generally, is everything Muhammad did considered as desirable, expected and/or good behavior for Muslims nowadays?
There is various views in this issue, some says they can be married before puberty age, and even have intercourse with them if they are 9 regardless of them being mature of not. Another says that marriage is valid for young but intercourse should not happen before "they are ready" for it. However, there is no clear cut ruling on what defines when a girl is "ready".
Other scholars noted that it is preferable for fathers to refuse marrying their daughters before reaching puberty.
Just a glance of this tells you there is really no clear cut on the matter, however, I would personally not accept such thing unless they reach puberty.
A side point, the doings and sayings of prophet Muhammad PBUH should be followed, but there are things that we just can't, like marrying more than 4 at a time. Another point, Muhammad PBUH married also women when they where much more older. So, what? Should we marry very young girls or really old women? It is not specified.
You can't say that those who accept marriage for their young girls are wrong, and you can't really say that those who oppose young girls marriage are wrong either. Law then comes to place.
No, it is not immoral (why should it be immoral? marrying is one thing and sexual intercourse is another thing )
No, not all things Muhammad did are desirable, for example, he traveled on a camel, but we must not desire to travel on a camel.
The version of the story mentioned in this question is actually very debatable. A lot of scholars believe that Lady Aisha was much older than that; more like 18 or 19 years of age.
Anyway, I think this subject needs more research. And to answer your question; I personally believe that marrying underage girls is something that is HIGHLY immoral.