Coming of age story at No. 1

When Lydia Ko tees off just before 8 a. m. Friday, she’ll ( A) be the leader of the Swinging Skirts LPGA Classic and ( B) be playing for the first time in which her age matches the number of holes in a round.

Yes, the world’s topranked player and defending champion at Lake Merced turns 18 on Friday. She crafted a 5- under- par 67 Thursday to take a one- stroke lead.

She doesn’t expect much to be different in her first round as an adult.

“I won’t change it just because I’m one year older,” Ko said. “Really, I’m only one day older.”

Ko said she doesn’t have big plans for her birthday celebration; she’s going to dinner with some players.

Many players talked about how difficult Lake Merced is, but 27 of them broke par Thursday.

“I think the scores are really good for how tough it was,” Ko said. Lewis’ crazy round: Stacy Lewis, who finished second to Ko in last year’s Swinging Skirts, was in a four- way tie for fifth after a 69. Lewis had five bogeys, six pars, six birdies and an eagle.

“It was probably one of the craziest rounds of golf I’ve ever played,” Lewis said.

The world’s thirdranked player was 1- over through 10 before holing out a 133- yard 8- iron for an eagle at the par- 4 11th. She birdied her final three holes.

“It could have really gone sideways today,” Lewis said. Briefly: Speaking of a round that could have really gone sideways: Brittany Lincicome was 5- over after double- bogeying the 10th and still 4- over through 13. She proceeded to eagle the par- 5 14th and then ripped off birdies at Nos. 15, 16 and 17 to finish with a 71. ... Rough rounds for Bay Area natives: San Jose’s Christina Kim had a 79, Pleasanton’s Paula Creamer an 82.