1) It is extremely hard to become a lich. First, you'd have to reach level 47 (spectre, cannot rank further as necromancer unless you are undead). Then, you will have to complete an Imm-controlled lich quest, looking for reagents of the elixir. Getting this quest might be tough. Then, you will probably need some friends to help you on the lich/mummy quest. You can also complete an automated mummy quest to become a mummy. Becoming a mummy is easier than becoming a lich. If you make the elixir from wrong reagents, the becoming will fail. The becoming is always dangerous and it has a chance to fail. If you fail in the becoming process, your character will die to old age, as he gave away his life to become undead.

2) If the highly unlikely event that you become undead will succeed, you don't have to worry about age death. Con loss will be a possibility, as undead lose one full con point for every death. Liches and mummies are powerful though, so you can avoid dying with more ease.

Yes, basically you can live forever as undead, but sooner or later, the undead delete when they get bored of their characters.

But seriously, play a character long and you find that age death isn't really an issue. When my character died to age death, I was basically ready for it. Most people delete before age death anyway.