So they didn't know how old he is so they just winged it -
"The name "Dostoevsky" is derived from the village of Dostoev in the Minsk region. No fewer than four nationalities go into the Dostoevsky family tree.. Belarusian, Russian, Ukrainian and Polish. The result being frankly as multinational as you could get at the time-. The Rtishchevs were Russian, the setting was Belarusian, the suffix "sky" is predominantly Polish, and over the years some of the Dostoevskys moved and settled in the Ukraine, while others like Fyodor Mikhailovici's branch ended up in Moscow.
"The Dostoevsky lineage presents a fascinating and unusual mixture of nationalities: In a family where the father was Lithuanian, the mother Ukrainian, there was a cult of Russian literature and history, the cult of reading. The atmosphere was one of devotion to the spoken word, and it is precisely this which above all else that shaped the author's creative make-up. Hence, Dostoevsky's Russianness is a wholly cultural rather than ethnic phenomenon" (Ludmila Saraskima)"
-Ignat Avsey on Dostoevsky's life
I can write more from this if you'd like in the future-