So what is the answer?
Leatherback was right and I'm surprised he guessed it. The tree looks 2, 3 years old max. My other pines are lots bigger (black pines, not white).
I experimented and used different pot sizes and soil mixes. This one has hardly grown and indeed, it is now not too healthy anymore.
It was planted in 2014 in a small pot in akadama .
Bonsai are not created by planting seeds in small pots in bonsai soil and trying to keep them small.