Kick-Ass is getting married ... and having a baby ... with a woman twice his age. Aaron Johnson, the 19-year-old title star of last weekend's No. 1 film, tells PEOPLE he and British artist-director Sam Taylor-Wood are having their first child together this summer.

Taylor-Wood, 43, is a whopping 24 years older than the young actor and already has two kids from a previous marriage. Johnson says he's "excited" about fatherhood and isn't even a wee bit nervous.

"It's not scary," he said. "I'm already a step-dad to [Taylor-Wood's kids, Angelica, 13, and Jessie, 3] anyway. So the nerves have sort of gone."