Does anyone know how old is christianity?

1970 years old if you count the crucifiction of Jesus, about 1950 if you count Paul, 1680 if you count the Council of Nicaea.

Outside observer point of view. At the time of Jesus, Christianity would be considered a Jewish sect. I would credit Paul with turing it into a separate religion. The council of Nicaea (among other things) chose Trinity over Arianism.

around 1965 years, considering that JC (if he existed) was martyred in 34 or 35 BCE, and his little cult elevated him to the level of a messiah or a savior.