I'm not quite sure I understand what you mean with "use inside webapp jars compiled on JDK 1.5". But it seems you mix up two things:
To run Tomcat 7.x, you'll need at least JRE or JDK 1.6.
However, you can use any JAR file, which was originally compiled for an earlier Java version as they are upwards compatible.
So if you're interested in using some older jar file, there's no problem.