My guess is Taki is some where around 20, and Claus is about 10 years older.
It's outright stated that their age has a 6 years gape; when Taki traveled away from home and went to Luckenwalde he was 18 years old. So, I'd assume that in the current timeline Taki's not that much older, and Klaus is halfway through his twenties.
How old is Klaus and Taki exactly...? I've been wondering...:since they have already met each other years ago, and klaus was already old enough to feel the way he did towards Taki, how old is he now?