Surprised but oddly calm, you realize that you weren't specific enough and it really was sheer luck that there weren't man sized ants running a muck through the park. So you take a second and a deep breath... "I wish I was three years older than my brother..." and after a seconds thought, " I also wish I was genetically set up to always have a lean athletic build regardless of how much I ate or how little I worked out... and did not have the genes to go bald."
A few seconds passed as the wishes came to fruition. You now had the body of a lean and muscular 22 year old. Though you were still in too small clothing, the pressure released significantly as you had already stretched all the fabric. Your once older brother and his friends are spending the next 23 hours and 58 minutes as ant slaves and you are supposed to be spending the afternoon with your family at this picnic. You should be freaking out, excited or terrified... both maybe?
You laid back, rubbing your new 6 pack, wondering why you were so calm about everything. Really, you just watched your older brother devolve... you reached up and touched you face, feeling a few days of stubble growth as though you had always been an adult. "That's really cool." you say aloud, sounding much like you did when you made yourself old enough to be your uncle, or dad...
Still lazily lying on the cool grass, you take a deep breath and wonder why your wishes are being granted, sure, its great, but you have wished on countess first stars of the night, the moon, the sun, pennies tossed into fountains, birthday candles, hell, you even sent wishes to God and a few times wished on plants in the forest, hoping that as they grew your wish would come true... why now?
Furthermore, you are eerily calm but your brother and his friends were freaking out as they turned to ants... you chuckled at the memory of his face... so how would any town member respond to you suddenly becoming 22, or 12 young men disappearing... without their clothes. You aren't worried, but even as your attitude begins to alarm you, a cool breeze blows over you and the concern is pushed aside.
"I wish I knew, in the simplest of terms, why my wishes are suddenly coming true." And with that you suddenly became aware of the fact that there was an enchanted wishing stone, not even a foot under the earth where you had been laying your head. You knew it was put there last night, and you knew that you were now its rightful owner as the previous owner abandoned it when their wish was fulfilled. (Starting your life over sorta makes you forget everything.)
You began to try to move to examine the earth under where you were laying your head but suddenly became acutely aware of how tight your clothes were... "Ugh, I wish my clothes were appropriate for me." A second later the tightness you had been feeling went away and you were wearing a graphic T-shirt and shorts.
You rolled over and felt at the ground finding a soft spot of freshly upturned earth. Pulling at the clod of dirt that was placed on top, it came up easily to reveal a bag, in the bag you found small gems and a shiny rock which seemed worthless compared to the gems. Grasping it, was anti-climactic as it felt like any other cool stone...
You don't know how people would respond to you in your new adult body, maybe you should wish to know if reality had changed and nobody would miss your brother or think you being 22 is strange. Maybe you should wish THAT reality changed or always would change unless you specify otherwise. Maybe wish that you were back to your normal self? Or you could just walk back to the party and see what happens, if people recognize you? Or try out the bar across the street... you could have alcohol before your brother, that's something you never expected... well, legally at least. Wishing for a book detailing how the stone works would be smart.