Information can be found Here
Quoted from the second link:
"Better yet, the Minecraft launcher automatically downloads the correct version
for your system (all those players inadvertently using Java 32-bit on their 64-bit machines should see significant performance boosts thanks to this) and automatically updates it. All this and you’ll never be prompted to install the Ask Toolbar again."
If it's not smart enough to know it needs an older version, can always install the older version and the newer one, and direct MC via the JVM arguments as has been discussed.