We just bought a 41 yr old home. The hot water only lasts for 5 to 7 minutes before it becomes tepid, at best. My husband found out that we have a tankless water heater. Is that our problem or could it be something else. Please help us. Thank you.

Tankless water heaters are notorious for that. You may be able to confirm what demand it is supposed to be able to keep up with from the owner manual.

Check the settings also, it may just be set too low.