In my work, I often come across sentence structures that on face value look illogical.
Now, the dictionary defines 從小 as “since childhood” or “from childhood”, and that’s what seems to be implied at the literal level. But “since childhood” implies “from that time all the way up to now, and possibly into the future”. But how can one continuously start to learn the piano throughout all that time? Starting is something done instantaenously.
So what is literally stated as “Since childhood I have started learning the piano” instead means “Since childhood I have been learning the piano”, or “As a child I started learning the piano”. (In practice, I would generally recast the sentence as “I started learning the piano as a child.”)
So it’s best to ignore what the dictionary says and what face-value logic would suggest, and interpret “從小” as meaning either “since childhood” or “as a child” depending on the context.
Another illogical construction I commonly see is 由於… , 讓 … This leads to a missing subject. But then this is Chinese: we don’t need no stinking subjects!
Example: 由於我的經驗, 讓我學到 X.
Literally: “Because of my experience, enabled me to learn X.” Perhaps better rendered as “My experience taught me X”, or “From my experience I learned X”.
I wouldn’t write a sentence like that either, but people here in Taiwan (no idea about the mainland) do it all the time… so much so that on a purely pragmatic level I’ve come to accept it as a valid sentence construction.
由於‧‧‧，讓 is considered ungrammatical and a common 「病句」 construct.
I see nothing wrong with your 從小 example. 從 is just a marker for indicating a starting point, that point being childhood (小). To be more verbose, your example means “I started learning piano from a particular point in time, that starting point being childhood”. 從 is often translated correctly as ‘since’ or ‘from’, but in this instance, using those translations make the English sound illogical. The words ‘since’ and ‘from’, when used temporally, indicates an on-going event unless the sentence specifies otherwise with an ‘until’ (or other similar contructs). 從 doesn’t necessarily have an ‘on-going’ implication.
Therefore, I think “I started learning piano as a child.” is the best translation. In both the Chinese and English sentences, only the starting point is mentioned. There’s no mention of whether the learning had stopped or is still on-going and cannot be ascertained in either language unless from context.