Mint 12 LXDE and Mint 9 LXDE end of life (EOL) is in April 2013 so you can continue to run Mint with LXDE for a few more months. The system should be usable after EOL but will no longer receive security updates or bug fixes and the repositories may eventually move or close.
There are alternatives, use XFCE instead of LXDE, install LXDE on top of another edition, make a community Mint 13 LXDE edition or move to another LXDE or light distro.
Moving from LXDE to XFCE it's not much work and LMDE XFCE is light, not as much as LXDE but close. When I tested Mint 12 LXDE against LMDE XFCE to make a more complete RAM tuning guide
in PDF format I was surprised by how light XFCE can be if properly implemented. Another option is Mint 13 XFCE LTS, it's slower and heavier than LMDE XFCE but is based on Ubuntu instead of Debian and has support until 2017.
Another option is Installing LXDE on top of another Mint edition. It's probably the best option if you have the time to configure LXDE to your liking. If you like, make a howto and share with the community.
Moving away from Mint is also a option. Lubuntu 12.04 is a very good option, you only need to make it look and feel more like Mint and the beta 1 for Lubuntu 12.10 is only 6 weeks away. Debian 6 LXDE or Debian testing LXDE is another option and if moving away from LXDE is not a problem you can try Bodhi (E17), Crunchbang (Openbox) or AntiX (IceWM/Fluxbox/etc). There are many Ubuntu/Debian based light distros.
The best option for the community would be if the users that want to keep Mint LXDE alive could come together and build the Mint 13 LXDE Community Edition. It's more work but could be fun. Anyone interested?