Sharon Osbourne doesn't think Justin Bieber will have a long career.

The showbiz matriarch thinks the 18-year-old singer is a ''great little guy'' but she doesn't know how long his success will last now his fans are growing older.

She said: ''He's a really great little guy. But that's his problem - he's a little guy.

''His fans are growing up, they're 18 and 19 now ... they're young women. And he still looks like that little boy!

''He needs some hair, or something ... I just don't think, from my gut - maybe my gut's wrong because it's not 100 per cent - but I don't think he's going to stand the test of time.''

However, Sharon - who managed her husband Ozzy's rock career - doesn't feel sorry for the 'Believe' hitmaker if he career does come to a premature end, because he has made ''oodles'' of money already.

She added: ''But you know what, I don't feel sorry for him, because he's made oodles and he's a fabulously nice person and he'll go on to do something else.''

Despite Sharon's attack, Justin insists he is ''not worried''.

He tweeted: ''Doubt me ... I like it. Not worried Sharon.''