I'm 38 and have been playing almost 6 months and have been taking lessons for the last month. I'm not sure a "curve" applies to learning an instrument. I myself have vowed not to get in a hurry to be good, just take it as it comes. As far as a bass goes, go play a bunch a different kinds and buy what feels good to your fingers and sounds good to your ears. I practice hours a day on both bass and drums, it's tough because drum rolls and basic rudiments kill my fingers as much as slow scale practice and basic neck work on the bass. In the end though my fingers are getting strong very quickly and that helps with both stick work and bass work
Bottom line: Find a good bass teacher in your area, get a decent bass and practice as much as you can. I promise there will be days when you can play well and days when your fingers just don't want to cooperate. Stick with it during the bad days and don't quit because the next day will be better.
Good luck....
