Having monitored this on a one airline basis for over 15 years, it is interesting to note that any so called retirement bulge is evened out by other perhaps less obvious factors.
Not all pilots retire at 55. Some continue to 60 with a progressive level of attrition in between. A proportion of pilots retire before 55 on health grounds.
Pilot recruitment is undertaken at various ranges commensurate with the levels of experience being sought. Other factors affecting the input are pension cost considerations, time to command requirements and market availability.
All of these factors combine to effectively smooth out the airlines requirement. Predictions based simply on retirement dates are likely to be very distorted in reality.