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 prior to installation.

2. Load Capacity: The roof must have sufficient load capacity to support the additional 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 roofing materials, but some materials may require additional 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 above 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.