Adults have been dressing younger and younger in recent years – sharing in teenage trends from low-rise jeans to this season’s flowy bohemian skirts. So perhaps it was only a matter of time before the line between adult and children’s clothing styles was blurred even further. As even the most cursory stroll through the Brownstone Brooklyn Baby Neighborhoods makes clear, studiously casual moms and dads are now styling their offspring as mini-hipsters. The days of Peter Pan collars and Little Lord Fauntleroy suits for children are long gone: Toddlers are sporting T-shirts bearing band logos and witty pop-culture references before they’re even old enough to talk.

Not surprisingly, a crop of boutiques has sprung up to both feed and fuel the demand for sufficiently cool baby clothes. I headed out recently to check them out.

My first stop was Park Slope’s Baby Bird (428 Seventh Ave., Brooklyn, 718-788-4506). From the moment I announced I was pregnant, people had been telling me to go to Baby Bird, and as soon as I entered this boutique, I could see why. The stylish clothes were really nice there – just the kind of things I’d buy for myself, if I could justify spending $32 on T-shirts. And there was a sale on, with summer items up to 50% off. Promising finds included very Paul Smith-looking men’s-style shirts for toddler boys by Imps and Elves (were $76, now $38) and Le Tigre Polo shirts (now $14-$17), for the 2-year-old who wants to dress like Seth Cohen of “The O.C.”

The T-shirts at Baby Bird almost never go on sale. “There’s no need to mark them down,” a helpful British saleswoman told me. “We can hardly keep them in here.” Popular styles include Baby Bird T-shirts ($32) made by artist Matt Walker, which feature crude but whimsical drawings such as a monkey riding a bicycle. Other big sellers are the Rock ‘n’ Roll T-shirts (also $32) bearing names of bands like the Ramones and the Sex Pistols, or the Rolling Stones’ iconic lips. And, just in case your baby is already a fan of “Charlie’s Angels,” there is a T-shirt with a classic Farrah Fawcett photo. “Parents like them because they’re the kind of T-shirts they wear, too,” the saleswoman said.

A similar ironic vintage vein was to be found at Area Yoga & Baby (252 Smith St., Brooklyn, 718-246-9453). The store, which opened less than a year ago, stocks T-shirts by local brand Bobo Brooklyn ($26.50) featuring sewn-on designs from vintage fabrics. Popular ones include kid characters such as Snoopy and E.T., as well as less obviously baby-oriented images like the Hulk, “Star Wars,” and “The Dukes of Hazzard.”

“I think the vintage look really appeals to moms and dads,” a saleswoman told me. “They see the ‘Star Wars’ stuff and it looks like their ‘Star Wars’ stuff – the styles they grew up with.”

Silk-screened items by Monsters With Sideburns at the store include a onesie featuring a bottle that says, “100% Organic, Bottled by Mom” and a T-shirt with a detailed drawing of headphones ($33).

Also on offer at Area are T-shirts and pants by Splendid. The label seems pricey for baby-wear: Rompers cost $42. According to the saleswoman, however, many neighborhood mothers leap to buy their babies Splendid separates. The mothers themselves “have Splendid T-shirts they got at Barneys,” she told me.”So they know how great and soft they are.”

Just up the street, Refinery (254 Smith St., Brooklyn, 718-643-7861) puts a different spin on babies dressing in their parents’ clothing. The shop, long known for its distinctive handbags, also carries vintage baby clothes from the 1970s. “The owner collects them,” said a friendly woman manning the shop. “Every so often she’ll come in with a few new items.” Offerings tend to be seasonal. At the time of my visit, they included a watermelon seersucker romper ($17) and a Carter’s rainbow colored jacket ($20).

In addition to the vintage clothes, the shop also stocks baby-, toddler-, and children-size versions of the ubiquitous “718” T-shirt, starting in a newborn onesie ($18; infant T’s are $15, youth raglan shirts are $18). “We sell as many baby ‘718’ shirts as we do adult ones,” the saleswoman told me.

Cool baby T-shirts are also big sellers at Urban Monster (396 Atlantic Ave,. Brooklyn, 718-855-6400). Popular this summer are shirts and onesies with the golden-eagle Trans Am Firebird ($27), the store’s owner said. The shop also stocks some items that put a cute spin on the trend of baby rocker shirts: an AC/DC-style black T-shirt says AB/CD, complete with a lightning rod in the middle, in silver letters, while a Kiss-inspired one says “Kids” and features cartoon baby versions of Gene Simmons and the gang ($28). Finally, there is a ($27) T-shirt that sums up this new baby clothes aesthetic. Available in sizes that start at birth, it says “Half Pint Hipster.”