You're right that length is an important factor. Each bit doubles the possible options because there are two symbols (0 & 1). Generally, the possibilities are S^P where S is symbols, P is positions, and ^ raises S to the power P. Except you're not using bits, you're using characters so if you lop of symbols, uppercase, and numbers using only lower case you lose possibilities (reducing S). The number of symbols in printable ASCII is 95 but there are only 26 lowercase letters. See how that changes the number of possibilities:
26^1 = 26 : 26^2 = __676 : 26^3 = _17,576
95^1 = 95 : 95^2 = 9,025 : 95^3 = 857,375
You can see that allowing more characters greatly increases the impact of length.
Your mentor is partly right but your take away papers over the impact of the base.
The passphrase approach [0] is optimized for human memory and I would suggest that for your master password in a password manager but for generated passwords drawing from the greatest set of characters increases security.
26^1 = 26 : 26^2 = __676 : 26^3 = _17,576
95^1 = 95 : 95^2 = 9,025 : 95^3 = 857,375
You can see that allowing more characters greatly increases the impact of length.
Your mentor is partly right but your take away papers over the impact of the base.
The passphrase approach [0] is optimized for human memory and I would suggest that for your master password in a password manager but for generated passwords drawing from the greatest set of characters increases security.
[0] https://xkcd.com/936/