Please settle a dispute

[KungFuPandaWorksOut20]

My husband is born in 1991, I said he's on his 4th decade. My logic being
1991
2000
2010
2020

That's 4 decades in my maths (although my maths isn't the greatest)

He's saying he's living his third decade not in his 4th because he's not 30 yet.

So wise mumsnet can you please shed some light on who is right. Or the correct answer if we are both wrong.

He's under 30, therefore he's in his third decade.

He is right

He was born in the 90s, and lived through the 00s, 10s and now the 20s so he has lived during four decades but not lived for four decades.

Next year after his birthday he will be in his 4th decade. He has lived in 4 decade but that doesn't mean he's in his 4th decade. I have lived in two centuries but it doesn't mean I am in my 2nd century.

He not on his fourth decade he’s lived thro four decades he’s on is third decade

He has lived in 4 decades but he has not lived for 3 decades yet so can’t be in his 4th.

I'm on my second millennium.

Third decade.

When you reach 30 you enter your 4th decade.

@malmi - you win!!