.mp2 - usually mean Video data using MPEG-2 video, which is formally called as ISO/IEC 13818-2.
.mp3 - usually mean MPEG-1 Audio Layer 3 which is formally called as ISO/IEC 11172-3.
Therefore MP3 is older than MP2 because MPEG-1 is older than MPEG-2. And, MP2 usually mean video file and MP3 is audio.
There exist Layer I and Layer II of 11172-3. But those are not widely used recently and they are not called as MP1 nor MP2.
