kagami and hiruzen were about 20 years old when tobirama died....when hiruzen was about 60 when minato died. at 65-70 shishu seems to be 18-23 years old. so 65-18= 47 meaning kagami was likely in his 40s when shishu was born. even if shishu was around 30 years old, kagami would had him at 35 years old, which is an acceptable time to have kids...
There is NOTHING comflicting with the timeline for shishu being the son of kagami being strange.