Im 24 and have a 4 year old daughter so I totally get where you are coming from. I too at times feel a little bummed that Im being a 40 yr old when Im only 24.. I do get frustrated at times when everyone around me who is my age are travelling the world, finishing their uni degrees, clubbing each weekend, shopping etc... I think we all feel like that.. its only natural. Dont feel bad.
I have days where I stay at home in my pjs and play barbies and watch numerous episodes of Dora the Explorer and you know what - thats ok....I have a beautiful, healthy little girl and I wouldnt trade her for anything. Our children are going to be all grown up while we're still fairly young so whatever you feel you are missing out on you can catchup on later in life. Life isnt about getting drunk every weekend - I feel sorry for young mums that feel the need to do that each weekend and dump the kids on their childs father or grandparents.. My daughters dad has her every second weekend.. some weekends that he has her I dont go out at all..I might just stay in and paint my nails and be in bed at 8pm lol - I guess as you grow up and your child gets older, it gets easier and you adjust to it all and you slowly forget about what everyone else your age is doing.
But yeah -- I know how you feel :0( Things will get better, every week theres always a bad day or 2 but theres always a 100 other days that are great and make up for the bad ones ;0)