While a 70mm bb shell is typical of Italian bicycles, some Japanese brands did use them too, so you really should check for threading standard markings on the cups. The Altus LT derailleurs came out in 1980, so it is almost certainly early 1980s as previously suggested by BG. While it's possible to get a close reading on the age from component date codes, if there is a serial number we may be able to determine the exact year and manufacturer. Overall, it appears upper entry level. I suspect it is hi-tensile steel. Baring a tubing decal, the best indicator of frame level is the seat post diameter.