Something is missing in the list of calculations. For it to work right, the number you need to have at the end of step 5 is 2010 if you already had your b day, or 2009 if you have not had your birthday.
It would work if you added a step 5b, where you subtract 100*(number of chickens)
A= ((C5*2)+5)*50)+1760
B= ((C5*2+5)*50)+1759
Adjusted:
A2=((C5*2)+5)*50)+1760-(C5*100)
B2 = ((C5*2+5)*50)+1759-(C5*100)
Example with B day 1986 that has occurred this year
A B A2 B2
# chick+b day -b day w/o 5b w/5b
0 2010 2009 24 24
1 2110 2009 124 24
2 2210 2009 224 24
3 2310 2009 324 24
4 2410 2009 424 24
5 2510 2009 524 24
6 2610 2009 624 24
7 2710 2009 724 24
8 2810 2009 824 24
9 2910 2009 924 24
10 3010 2009 1024 24
11 3110 2009 1124 24
12 3210 2009 1224 24
13 3310 2009 1324 24
14 3410 2009 1424 24
15 3510 2009 1524 24
16 3610 2009 1624 24
17 3710 2009 1724 24
18 3810 2009 1824 24
19 3910 2009 1924 24
20 4010 2009 2024 24
21 4110 2009 2124 24
22 4210 2009 2224 24
23 4310 2009 2324 24
The adjusted method will give you your exact age even if you are over 100.