As a sort of off-topic but related to this, you should be generating segwit compatible vanity addresses now. Look around the forum for them. They spit out addresses that start with bc1 or 3.
I don't use them anymore as much, I just stick to random ones. You can have seed words for those too.
If the private keys you have begin with a 5, then your software will work, but it is outdated. Compressed private keys begin with L or K, but any address that begins with a 1 is also a legacy address.
There are some more restricted characters that can't be used when generating bc1 addresses: 0, O, l (lowercase L), I (uppercase I); those four are expected because they're not in base58, but also, 1, b, i, o can't be inside a bc1 vanity address according to 1miau:
Nested SegWit (3...) has to start with an uppercase letter or a number. For example
3Fery works but
3fery not.
Native SegWit (bc1q...) has some more "restrictions" and in addition to O, 0, I and l the letters / numbers 1 (except the 1 in bc1q), b, i and o won't work. So sad that I can't create
bc1qmiau...
3 vanity address can't start with a lowercase letter, and I asked about this, and the reply I got is it also can only have prefixes between
31h1 and 3R2c, so 3111 and 3SSS are also invalid segwit prefixes to search for.
So I think that is why people still generate legacy addresses even though they have higher fees, because there are less restricted characters.