Certainly! Here's an updated version of the SPARQL query that includes a filter to fetch up to 50 Wikidata items representing people who identify as non-binary and are younger than 50 years old:

```
SELECT DISTINCT ?person ?personLabel ?genderLabel ?birthDate
WHERE {
  ?person wdt:P31 wd:Q5;  # Instance of human
          wdt:P21 wd:Q48270;  # Has gender identity as non-binary
          wdt:P569 ?birthDate;  # Has birth date
          rdfs:label ?personLabel.
  FILTER(LANG(?personLabel) = "en").
  
  OPTIONAL {
    ?person wdt:P21 ?gender.
    ?gender rdfs:label ?genderLabel.
    FILTER(LANG(?genderLabel) = "en").
  }
  
  FILTER(YEAR(NOW()) - YEAR(?birthDate) < 50)
}
LIMIT 50
```

In this updated query, the `wdt:P569 ?birthDate` statement retrieves the birth date of each person. The `FILTER(YEAR(NOW()) - YEAR(?birthDate) < 50)` statement filters the results to include only those individuals who are younger than 50 years old.

Feel free to adjust the age limit according to your requirements by modifying the value `50` in the `FILTER` condition.
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