とされる means "is considered", "is held to be": it reports a view that is generally accepted or officially stated, without saying whose it is. It is the quotation plus , the passive of , and follows a plain-form clause or a noun, with or without .
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These ruins are considered the oldest in the world.
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The crime is thought to have been committed late at night.
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He is regarded as a genius.
とされる belongs to news reports, academic writing and official documents. In conversation you say or .
とされる or とされている
describes a view that stands now, and is the more frequent form in reports. Plain とされる sounds more like a statement of record, and is the one you meet in headlines, reference books and definitions.
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This medicine is regarded as safe.
ににれいれい2200。
Learning Japanese is said to take about 2,200 hours.
A ruling or a judgement
In the past tense, とされた often reports a decision by someone with authority: an inquiry, a court, an organisation.
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The cause of the accident was found to be poor maintenance.
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His remarks were deemed a problem, and he was asked to resign.
The negative puts after と: , "is not considered".
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At the time, the act was not considered illegal.
とされる, と言われている, と考えられている
reports what people say, and works in speech.
reports a conclusion someone reasoned to. とされる is the most impersonal of the three: the view is simply on record.
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This temple is said to have been built more than 800 years ago.
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Dinosaurs are thought to have died out after a meteorite impact.
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This area is considered dangerous.
Lingaku