On interesuje się historią od dziecka.

Because the Polish verb is interesować się, which means to be interested in.

In Polish, many verbs naturally come with się, and you learn them as a unit. So here:

• interesować się = to be interested in
• on interesuje się historią = he is interested in history

Even though English does not use a reflexive word here, Polish does.

Because interesować się requires the instrumental case.

The noun historia changes like this:

• nominative: historia
• instrumental: historią

So:

• interesować się historią = to be interested in history

This is something you should memorize with the verb:

• interesować się + instrumental

It is the 3rd person singular present-tense form of interesować się.

Full present tense:

• ja interesuję się = I am interested in
• ty interesujesz się = you are interested in
• on/ona/ono interesuje się = he/she/it is interested in
• my interesujemy się
• wy interesujecie się
• oni/one interesują się

So on interesuje się means he is interested in.

Yes, it could be omitted.

Polish often leaves out subject pronouns when the verb already shows who the subject is. So both of these are correct:

• On interesuje się historią od dziecka.
• Interesuje się historią od dziecka.

Using on can add emphasis, contrast, or clarity.

For example:

• On interesuje się historią, a ona matematyką.
  = He is interested in history, and she is interested in math.

It means since childhood or from when he was a child.

Literally, od often means from/since, and dziecka is the genitive singular of dziecko.

So:

• od dziecka = since childhood
• od małego = since he was little
• od dzieciństwa = since childhood

In this sentence, od dziecka tells you how long this interest has existed.

Because the preposition od requires the genitive case in this expression.

Base form:

• dziecko = child

Genitive singular:

• dziecka

So:

• od dziecka = since childhood

This is a fixed and very common expression.

Yes, but it is slightly different in style.

• On interesuje się historią od dziecka = He has been interested in history since childhood.
• On jest zainteresowany historią od dziecka = He is interested in history since childhood.

The version with interesuje się sounds more natural for a lasting personal interest or hobby.
Jest zainteresowany can sometimes sound a bit more formal or temporary, depending on context.

So in this sentence, interesuje się is the most natural choice.

It mainly modifies the whole idea of being interested in history, and yes, it can move.

For example:

• On interesuje się historią od dziecka.
• Od dziecka on interesuje się historią.
• On od dziecka interesuje się historią.

All are understandable, but the most neutral and natural version is:

• On interesuje się historią od dziecka.

Polish word order is flexible, but changing it can change emphasis.

In this sentence, it means history as the subject or field of study.

Polish historia can sometimes mean:

• history = the past, the academic subject
• story = a narrative, depending on context

But with interesować się historią, the usual meaning is to be interested in history.

If you meant a story, the context would normally make that clear.

It is in the present tense, but with od dziecka it describes a situation that started in the past and continues now.

So although Polish uses the present form interesuje się, the full meaning is:

• he has been interested in history since childhood

This is very natural in Polish. English often uses has been interested, but Polish simply uses the present tense plus a time expression like od dziecka.

Yes. Both are correct and very similar.

• od dziecka = since childhood / since he was a child
• od dzieciństwa = since childhood

Od dziecka is very common and conversational.
Od dzieciństwa can sound a little more formal or abstract.

So both work:

• On interesuje się historią od dziecka.
• On interesuje się historią od dzieciństwa.

A very useful pattern is:

• interesować się + instrumental
• od + genitive in time expressions like od dziecka

So from this sentence, you can learn:

• interesować się historią
• interesować się muzyką
• interesować się sportem

and

• od dziecka
• od wczoraj
• od dawna

That makes this sentence a great model for building your own.