Newbie HR guy - how to best forecast retirements in company

Hi - I am a HR professional looking to self learn statistical modeling for new responsibilities at work..Need to forecast no. of employees who may retire next 10 years. What would be simple way to forecast for this? I have historical retirements data broken by age groups (50-55 etc).

Have tried couple of approaches that give varying results.

1. Based on avg age of employees retired in past for each business group (this give a large no. of employees to retire since it assumes employee retire after reaching the avg age.) ~5k at end of 10 yrs
2. Using per yr retirement rate by age range and business group. Gives only ~1.5k retirement at end of 10 yrs. Looking at data, the percent I used to calculate from prior yr is much lesser at employees in higher age bracket 70 above since historically the no of employees at these higher age range is lesser as employees retire or quit before reaching this age..

Would greatly appreciate if you could share thoughts on considering mathematical model than historical avg rates or if there are any other suggestions perhaps to use sample data differently applying simple historical averages.

Thank you.