He started out in KDL as a kid who just really wanted food. There's no "thousands of years old" about that. Mixing game canon and anime canon is just the worst.
I couldn't give an exact age, especially given stuff like you don't really know how long a year is on pop star, you don't know the average life span for any species on dreamland, let alone Kirby's species (which only has one known member - two if you count Keeby and three if you're really dumb and count meta knight) and you don't know how much time the series spans. I will say he seems to start out as young and pretty dumb but ends up becoming decently experienced and forging friendships. He's still obviously youngish at heart, but it would be pretty hard not to be young at heart in Dreamland, so that doesn't really say much about his actual age.