So, the question is if it would be fine and ethic to set the "Date of
death" to "unknown" on the base of an old date of birth.
And about the biography of living persons, I found this 
Deceased persons, corporations, or groups of personsRecently dead or
Anyone born within the past 115 years (on or after 19 September 1904) is
covered by this policy unless a reliable source has confirmed their death.
Generally, this policy does not apply to material concerning people who are
confirmed dead by reliable sources. The only exception would be for people
who have recently died, in which case the policy can extend for an
indeterminate period beyond the date of death—six months, one year, two
years at the outside. Such extensions would apply particularly to
contentious or questionable material about the dead that has implications
for their living relatives and friends, such as in the case of a possible
suicide or a particularly gruesome crime. *Even absent confirmation of
death, for the purposes of this policy anyone born more than 115 years ago
is presumed dead* *unless* reliable sources confirm the person to have been
living within the past two years. If the date of birth is unknown, editors
should use reasonable judgement to infer—from dates of events noted in the
article—if it is plausible that the person was born within the last 115
years and is therefore covered by this policy.
This would support the set of "Date of death" to "unknown" on the base
the "Date of birth". It remains hard to verify typo errors, but we are
doing our best to verify the data of the several wikiprojects.
The property set would become effective if done in mass by a bot or similar.
By the way, I would extend be period to 122 years 
Il giorno gio 19 set 2019 alle ore 21:29 Andy Mabbett <
andy(a)pigsonthewing.org.uk> ha scritto:
On Sat, 7 Sep 2019 at 07:53, Fabrizio Carrai
I found athletes with the "Date of
born" but with NO "date of death".
So a query on the age show me athletes up to 149 years old.
Since the oldest know person was 122, what about to set "date of
death = unknown value" for all the persons resulting older such age ?
Yes, but check that the date of birth isn't a typo (i.e. 1875 instead
of 1975; or 1894 instead of 1984).
Showing a living person as being dead would be a serious breach of the
Wikidata mailing list