How old is my Sears 410 shotgun model?

If it has no serial number, it is definitely before 1968. As of 1968, the Gun Control Act required serial numbers.

These guns were made on contract usually by Stevens, but sometimes by other companies. After 1968, the serial number can be compared to online archives, however, records aren't exact and it can be hard to narrow it down beyond a range of several years.

If you search online, you can probably find a similar gun on an auction site, fan site or interest site to compare to. Similar shapes and finishes were used in blocks over several years, and that will help narrow it down.