well he didnt lose to dooku, dooku was already knowing he wouldnt win it was a matter of time befor yoda killed him, so dooku toppled that big metal pole or whatever it was, to distract yoda long enough to escape, dooku knew yoda would save obi-wan and anakin from being crushed.
and lastly, i know sidous was declared the victory between him and yoda, but did he truly win, no, he niether killed nor defeated yoda with force capability, sidious won in the fact that he had anakin and the clone army kill off the jedi, after sidious and yoda fight yoda escapes and is taken away by bail organa, yoda tells him he failed, yes he failed to stop the sith from taking over but was because he was so heatbroken by losing his children, and that is the way i see this, that yoda just gave up, he was too evenly matched with sidious to try and restart the fight again.
and the age issue, yoda maybe nearly 900 years old, but he still could whip ass and take names.