How to search in redis for hash keys?

You cannot. Redis is a key-value store, not a relational database.

In order to search for a specific data, you need to build an access path to this data. For instance, to get the users having age = 25, you need to build an index to map the age values to users. It can be done with a set. This is the same for the name.

Once you have sets for age and name, you can search users by intersecting the sets. For example:

# Add 3 users
hmset user:1 user_name lee age 21
hmset user:2 user_name david age 25
hmset user:3 user_name chris age 25

# Maintain age index
sadd age:21 1
sadd age:25 2 3

# Maintain name index
sadd name:lee 1
sadd name:david 2
sadd name:chris 3

# Get the ID of users having age = 25 and name = lee
sinter age:25 name:lee
  -> will return an empty set