If you grow back a beard immediately, it will take the same amount of time.

Now for me, I grew my first beard at age 23 and took 6 weeks to come in. My next attempt was delayed due to multiple reasons and I was age 25 for that and it took 4 weeks to come in, so those two years added some more maturity to the growth rate of my own beard, but this in not the typical thing to happen.