This information can be found on the developer's web site. Check out the FAQ and the release notes sections. In general, small distros will come with a 2.4 series kernel (2.4.29 is likely) simply because it is smaller then the 2.6 series. However, this is not to say you cannot use a 2.6 kernel with puppy.
From the puppy linux release notes:
The kernel is still version 2.4.29, exactly the same as used from Puppy 1.0.4 - 1.0.7, except the default ramdisk size has been increased to 13824K. Puppy 1.0.8beta uses 2.4.31, however ndiswrapper does not work with it and we are forced to go back to 2.4.29 (see notes below).
For the future: please stick your location and your distro in your profile. Thanks.