The One Direction hunk is anticipating celebrating the milestone with his bandmate since their birthdays are only ''two weeks'' apart with Niall's on September 13 and Liam's on August 29.
He's excited about being able to have an alcoholic drink in the US where 1D - which also includes Harry Styles, Louis Tomlinson and Zayn Malik - will be performing as part of their 'Where We Are Tour'.
Talking on Irish radio station Spin 103.8, Niall said: ''I reckon myself and Liam are probably going to end up having a joint 21st because his birthday is about two weeks before mine.
''We'll have a few drinks there. We've been touring America for the past three years and not been able to touch a drop.''
Niall is also glad to be performing in his hometown of Dublin at Croke Park where they kick off the European leg of their tour on May 23 and has been missing the place since he's not had a chance to visit since Christmas.
He explained: ''Croker is going to be unbelievable, we're all saying it. It's been a while since we've been in Dublin last, I haven't been at home since Christmas.''
He also slapped a jet-lagged Taylor Hawkins.
The acclaimed performer had just cancelled touring due to a battle with cancer.
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