I did not know I could get energy credit for replacement windows and gas hot water heater. How young do they have to be for the credit?

[PaulaM]

The credits for energy efficient home improvements have expired (2016). 

All that is left is federal tax credits for energy efficiency improvements for solar energy systems (solar water heaters and solar panels). These federal tax credits remain in effect through December 2021. For more information on solar energy systems, see below.

https://www.energystar.gov/about/federal_tax_credits 

You may be eligible for additional tax credits or rebates in your region. Contact your local utilities company to find out, or visit dsireusa.org to search the DSIRE Database of State Incentives for Renewables and Efficiency®

[edited 1/26/18 | 7:57 am PST]