Hafþór Júlíus Björnsson plays The Mountain. He was born in 1988. He is currently 25 years old.
Thomas Brodie-Sangster plays Jojen Reed. He was born in 1990. He is currently 24 years old.
These two people are only a year apart in age.
And while we're at it, this is what Thomas Brodie-Sangster looked like when he was the little kid in Love Actually.
Just let that all sink in.
(h/t this very very observant Tumblr user)