There is no strict age limit for installing solar panels on a roof. Solar panels can generally be placed on roofs, regardless of the roof's age. However, the condition and load-bearing capacity of the roof are important considerations.

Important factors to consider include:

1. Roof condition: The roof must be in good condition. Any damage, leaks, or other issues should be addressed before installation.

2. Load-bearing capacity: The roof must have sufficient load-bearing capacity to support the extra weight of the solar panels and the mounting system. In some cases, structural reinforcement may be necessary.

3. Roof material: Solar panels can be installed on various types of roof materials, but some materials may require extra precautions or installation methods.

4. Orientation and slope: The ideal orientation and slope of the roof for optimal solar energy generation should be considered.

If the roof is in good condition and meets the aforementioned criteria, solar panels can be installed on almost any type of roof. However, it is always wise to consult a professional solar installer for a thorough roof assessment and to ensure that the installation can be carried out safely and efficiently.