Here is the circuit for a slow start device for a torch. It can extend bulb life by reducing the initial current surge at turn on.

The turn-on speed is controlled by R1 and C1.

The rate of change of bulb voltage will be about:-

Volts per second

I suggest R1 = 100 kohms and C1 = 10 microfarads

R2 can be just about anything. I suggest 10 kohms

Q2 is a small signal NPN transistor. A BC184 is fine.

The diode can be any small diode. A 1N4148 will do.

Q1 is the critical component. It is a power MOSFET, and it takes the bulb current, so the wires joining it to the bulb and battery must be large enough. The MOSFET must be a logic-level MOSFET, and the "On" resistance should be low, so that you don't loose too much voltage in it. I suggest a RF1S30N06LE.